DREDGING FOR METALS 223 



The tables empty into a chute of semicircular cross-section running 

 out over the stern of the dredge. The gold that has not passed 

 through the screen perforation onto the gold-saving tables is collected 

 in the sluice-box. This can be made 24 ft. long, 3 ft. wide and 18 in. 

 deep, fitted with ripples placed above mattings made of some fibrous 

 matter and discharging into the semicircular chute. 



The stones, gravel and other materials, after having been thor- 

 oughly washed out in the revolving screen, come out from its lower 

 end and by means of a conveyor are deposited at sufficient distance 

 and height for stacking purposes. Fig. 72 shows the side view 

 and plan of a placer dredge with revolving screen as built by the 

 Bucyrus Company of South Milwaukee, Wis. 



When the material is finely divided, instead of a revolving screen 

 shaking screens are employed. As in the dredge previously described, 

 the material is delivered into a hopper, through which it is conveyed 

 to shaking screens of sufficient size to handle the largest stones 

 that can be brought up in the buckets, and of sufficient length to 

 thoroughly screen and wash the material from a series of openings 

 above. Small streams of water are projected upon all parts of the 

 screen, washing and disintegrating the material before the finer 

 particles pass through to the gold-saving tables. The coarser material 

 is discharged at the lower end of the screen into a hopper leading 

 to a belt conveyor or tailings-stacker. The fine material passes 

 through the screens into a distributor, placed beneath the screens 

 and above the gold-saving tables. The object of this is to distribute 

 the material evenly over the tables. These tables are fitted with 

 ripples, or such other devices as may be best adapted to the char- 

 acter of the gold. The material which passes over the tables is 

 carried into sluice-boxes which lead aft to a point about 20 ft. 

 beyond the stern of the dredge. These sluice-boxes are also fitted 

 with ripples. Fig. 73 shows the plan and elevation of the placer 

 dredge with shaking screen built by the Bucyrus Company of South 

 Milwaukee, Wis. 



When the gravel or the gold-bearing material is very fine the 

 treatment by the sluice-box method is employed. Then the dredged 

 material in tipping over the upper tumbler of the ladder goes 

 through a drop plate, discharging into a long sluice which runs aft 

 for the length of the dredge and over the stem. The length of 

 the box will depend on the position of the tumbler framing, and 

 the height to which the material is to be stacked. As a general 



