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TUNNELS AND SLUICES.
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of
the blocks, and has given excellent results. The block-and-rock riffles
are not desirable for those sluices which have to be frequently cleaned
up.
Longitudinal Riffles.—In
some districts longitudinal riffles, made of scantling placed
lengthwise in the sluice, are preferred. At the Paragon Mine, Placer
County, where the banks contain many large boulders, the riffles are
made of 6-inch scantling 1-1/2 inches wide, 8 feet long, separated by blocks 1-1/2 inches wide; and an iron bar, 1 1/2 inches
wide and 1 inch deep and 8 feet long, is fastened on top of each
scantling. The grade of the Paragon sluices is 18 inches per 12-foot
box, and the width of the sluice is 44 inches.
Bed-Rock Riffles.—In
the tunnel of the North Bloomfield Mine the lower 6,000 feet are run
without a sluice, the bare bed-rock being used. Up to 1877, 7,000,-000
cubic yards were washed through the tunnel, and an examination at that
period showed that the tunnel had been deepened about 16 inches, and,
though the sides were worn smooth, troughs and holes were found
hollowed out at different places. A partial examination of the tunnel
made in the fall of 1882 showed the existence of many holes in the
bottom, in some instances 6 feet deep, but the wear on the entire line
may be said to average 3 feet, about 22,000,000 cubic yards of gravel
having passed through it.
On long sluice lines it is common to use several kinds of riffles.
Branch Sluices.—Where'
the topography of the country compels the building of branch sluices,
or a light dump requires the frequent change of the tailings
discharge, great care must be taken in constructing the connections
with the main sluice ; otherwise, in "turning into" and "turning out"
from a sluice, the gravel forms a bar either above or below the
junction.
Where heavy grades can be obtained no difficulty is encountered ; but where the inclination is slight, good
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