226 TUNNELS AND SLUICES.
In the 6-foot sluices of the North Bloomfield Mine, with a 4-1/2 per
cent, grade, the blocks, which are 13 inches deep and 20 inches square,
last for a run of 175,000 to 200,000 inches of water. At the Manzanita
and French Corral mines the sluices are 5 feet wide and have a grade of
4-3/4 per cent. The blocks, of the same size as the last, but of rather
poorer timber, have a life generally of 125,-000 to 150,000, sometimes
of only 100,000, inches of water.
At
La Grange, in 4-foot sluices on 2 per cent, grades, the blocks, 14
inches square and 8 inches deep, are estimated to last an average of
six months, during which time about 100,000 to 110,000 inches of water
are run over them.
After
each run the blocks are turned and replaced in the sluice, if not worn
down too much. A block reduced to 5, or at most 4, inches in d.epth is
considered unserviceable. In repaying with old blocks the edge worn
down the most is placed up-stream. As the blocks do not till the whole
width of the sluice, the alternate rows are fitted so as to break
joints.
Rock Riffles.—In
many localities stones instead of blocks are used for riffles, and
where heavy cement is washed the former are considered preferable on
account of their cheapness. At Smartsville they have been found to
serve fully as well as blocks, and are claimed to be cheaper. It must
be stated, however, that they are more costly to handle, as longer time
is required to clean up and repave the sluices.
The
stone riffles as quarried are of irregular size and shape, and are set
in the sluice with a slight tilt downstream. The hard rock used at the
Manzanita Mine, Sweetland, Nevada County, costs about $10 per box (14
feet long and 5 feet wide).
Blocks and Rocks.—A
system of riffles consisting of a row of blocks alternating with an
equal section of rocks has been found to work successfully. This
arrangement of the sluices reduces materially the wear and tear
