d
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. The wooden sluices in the tunnels already described
are often made double for the convenience of "cleaning up" one of them,
while the other remains in action. The process of cleaning up is performed
according to the quantity and richness of the material worked upon, at
intervals of twenty to forty days, and consists in removing the pavement
and blocks from the bed of the sluice, and then gathering all the amalgam
of gold and rich dirt collected, and replacing the locks in the same way
as at first. Advantage is taken on this occasion to reverse the position
of the blocks and stones when they are worn irregularly, or substitute new
ones for those which are worn through. The mechanical action of the
washing process on the blocks is of course very rapid and severe,
requiring complete renewal of them once in eight to ten weeks. Some miners
prefer a pavement of egg-shaped stones set like a cobble-stone flooring,
the gold being deposited in the interstices. Most of the sluiceways are,
however, paved with rectangular wooden blocks, with or without stones as
described. Standing at the mouth of one of the long tunnels in full
action, any person unaccustomed to the process is struck with
astonishment, amounting almost to terror, as the muddy mass sweeps onward,
bearing in its course the great rolling bowlders, which add their din to
the roar