DIFFERENT METHODS OF MINING. 81
namely,
the value of the claim, which could only be ascertained alter all the
principal expenses had been incurred. The losses in many instances were
very large, but in other cases the gains obtained in a short time were
so enormous as to throw around this class of work a fascination which
induced many to engage in it.
To obviate the necessity of turning the rivers Out of their
channels dredging machines have been buiit and used ; and the plan oi
sinking shafts on the banks and tunnelling (drifting; under the
surface of the bed has been suggested. Projects for working the river
channels (always supposed to contain enormous stores of hidden wealth)
are still proposed from time to time, but actual operations are not
common.
Ground-Sluicing. —
Ground-sluicing consists in treating the gold-bearing gravel, which is
excavated by pick and shovel, by washing it in trenches cut in the
bed-rock. it is similar to hydraulic mining, except that the water is
not used under pressure and often no wooden sluices are used below the
trenches, the rough natural rock serving for riffles. The lighter
material is removed by means of the water, while the heavier dirt
remaining behind is collected and worked in rockers. This process of
gold-washing was carried on by the Romans in the early part of the
Christian era.
Booming.—Booming
is simply ground-sluicing on a large scale, the only difference being
that instead of washing the gravel by means of a continuous stream of
water, the contents of the entire reservoir are discharged at once and
all the material which has been collected below it is swept into the
sluices. The rush of the water carries off the boulders and dirt,
leaving behind the heavy particles of gold and magnetic iron sands,
which are collected on bed-rock floors. Booming has been extensively
practised in California, Idaho, Montana, and Colorado. The
requirements for this kind of gold-mining are a sufficiently large
reservoir conveniently situated above the gravel de-
