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224 TUNNELS AND SLUICES.
To each box 15 pounds of nails are used—viz.:
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Riffles.—The
use of riffles dates back to the earliest days of gold-washing.
Blankets, hides with the hair turned uppermost, and grass sods were
employed by the primitive South American miners, and also steps cut in
the bare bed-rock. In California every variety has been tried, but
blocks and rocks are now generally used.
The
character of the riffle employed is dependent upon the length of the
sluice, while the length of the sluice, in turn, depends upon the
hardness of the gravel, and more especially upon the character of the
gold—scale gold, with large amounts of black sand and fine sulphur-ets,
escaping all riffles for long distances.
Block Riffles.—Block
riffles are square wooden blocks 8 to 13 inches deep, set on end in
rows across the sluice, with each row separated by a space of 1 to 1 1/2 inches. They are kept in position by riffle strips, 1 1/4 inches
thick by 2 or 3 inches wide, held crosswise on the bottom, between the
rows, by the side lining, and secured to the blocks by means of
headless nails. Block riffles are also set and firmly held in position
by means of soft pine wedges driven between the blocks and the sides of
the
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