DREDGING FOR METALS 



225 



rule the box is from 40 to 50 ft. long and varies in width from 3 to 

 6 ft., according to the class of materials to be treated. The pitch 

 depends on the same thing and is from 1 in 8 to 1 in 10. Fig. 74 

 shows a sluice-box as given in a paper by Messrs. E. S. and G. N. 

 Marks, reproduced in Engineering News, from which this description 

 is condensed. The bottom of the box is laid with perforated plates 

 and ripples for its entire length, the ripples overlying the fibrous 

 matter. It was found convenient to oise _as many different classes 

 of ripples as possible so as to alter the flowing of water and conse- 

 quently the material being treated is tossed about, and the gold 





FIG. 74. Placer Dredge with Sluice-box. 



which may be adhering to the stones has more chance of being 

 liberated. The classes of ripples used are angle iron, Venetians, 

 perforated plates and crimped or diamond-shaped. A return box 

 is sometimes used fitted as indicated in the figure. An opening is 

 made at the bottom of the box over which a perforated plate is 

 placed and all the material that passes under the plate goes out by 

 the return box behind the tail of the sluice run. The return box 

 serves two purposes: it either saves any light gold that has traveled 

 down the sluice-box or else it may be used to prevent a portion of 

 the water from falling on the tailings. Plan and elevation of a 

 ladder dredge with long sluice-box as used in Australia is given in 

 Fig. 75, reproduced from the paper of Messrs. Marks. 



