The grade in this tunnel is about 12 per cent.,
and the end of the tunnel is designed to be 170 feet of elevation, and
reaching to a point beneath the surface of the gravel which is being
operated upon, and where a shaft or incline is sunk to or through the bed
rock or gravel, until it intersects the tunnel.
The object of this laborious operation is obvious, as the long tunnel
becomes a sluiceway, and through the whole length of which sluice boxes
are laid, for the double motive of carrying off the material and saving
the gold, and for this purpose a trough of strong planks is placed in the
tunnel, 21/2 feet wide, and with sides high enough to contain the stream.
The pavement of the trough is generally laid of blocks of wood 6 inches in
thickness, cut across the grain, and placed on their ends, to the width of
the sluiceway. The wooden blocks are usually alternated with sections of
stone pavement, the stones being set endwise, and in the interstices
between the stones and wooden blocks quicksilver is distributed, and as
much as 2 tons of this metal is required to charge a long sluice. The
water in the canal is brought by aqueducts, or other means, to the head of
the mining ground, having an elevation of 100 to 200 ft. above the lowest
level of the mining ground, and is finally conveyed to it by iron pipes,
sometimes sustained on a strong incline of timber.
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