sluices.
The question of grade is therefore one of vital importance, and to
property investigate and determine this point great care and skill are
requisite. When the topography of the country admits of unlimited fall
the grade upon which the sluices are set should be regulated by the
character of the gravel. Where the wash is coarse and cemented,
requiring blasting, or where there is much pipe-clay, a heavy grade is
necessary. Strongly cemented gravel requires drops to break it up.
General Grade Axlopted.—Experience thus far has led to the adoption in most localities of what is called a 6 or 634-inch grade, meaning 6 or 6-1/2 inches
to the box 12 feet long, or, say, a 4 to 4-1/2 per cent, grade. In some
places, where large quantities of pipe-clay are washed off, 9 and
12-inch grades to the box are used (6 to 8 per cent.) In others, on
account of natural obstacles encountered, a 1 1/2 per cent, grade, or 2-1/2 to 3 inches per box of 16 feet, is used.
Light
gravel containing clay or earthy matter can be moved on an easier grade
and with less water than heavy gravel; nevertheless, when a 4-1/2 per
cent, grade can be obtained it is desirable, as it lessens the labor of
handling rocks and more material can be washed. Moreover, as light
gravel is generally poor in gold, this deficiency can be made up only
by washing large quantities. Light gravel requires that the water
should be run with sufficient force to carry off the rocks washed
through the sluice, and yet be in only sufficient volume to prevent t
he packing of black and heavy sand. If too much water is used by
superincumbent pressure the sand drops and packs the riffles.
The
best results are obtained with shallow streams on light grades. Coarse
gravel demands from four to seven per cent, grades and a proportionate
increase of water. In washing this heavy material the water in the
sluice should be dee]) enough (10 to 12 inches) to cover the largest
boulders ordinarily sent down.
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