Bowlders of 100 to 200
lb. in weight are dislodged and shot forward by the impetuous stream,
accompanied by masses of the harder cement which meet in the fall, and by
the concussion from the great bowlders the crushing and pulverizing agency
required is found to disintegrate it. The heavy banks, of 80 feet and
upward, are usually worked in two benches, the upper never being so rich
as the lower, and also less firm, and therefore worked away with greater
rapidity.
The lower section is much the more compact, as this stratum on the bed
rock being strongly cemented resists great pressure, and even sometimes
the full force of the streams of water, until it has been loosened by
gunpowder or other explosives. For this purpose adits are driven in on its
foundation-point of from 40 to 70 feet and more from the face of the bank,
and drifts are extended at right angles therefrom to a short distance on
each side of the adit, and in these drifts a large quantity of gunpowder
is placed (from 1 to 3 tons), and fired at one blast, having been
previously built in with masonry. And in this manner the compact
conglomerate is broken up, and then the water easily completes its work.
Sometimes in the soft, upper strata the systems of tunnel is extended, as
in a coal-mine, by cross alleys, leaving blocks which are afterward washed
away, and then the whole mass settles, and is disintegrated under the
influence of water.
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