82 DIFFERENT METHODS OF MINING.
posit,
and a dam for storing the water, so arranged that flood-gates can
quickly discharge the entire contents of the reservoir without damage
to the dam.
Pliny,
in his " Natural History," speaking of gold-washing, says: " When they
have reached the head of the fall, at the very brow of the mountain,
reservoirs are hollowed out a couple of hundred feet in length and
breadth, some ten feet in depth. In these reservoirs there are
generally five sluices left, about three feet square; so that the
moment the reservoir is filled the flood-gates are struck away, and the
torrent bursts forth with such a degree of violence as to roll outward
any fragments of rock which may obstruct its passage. When they have
reached the level ground, too, there is still another labor that awaits
them : trenches, known as ' agogae,' have to be dug for the passage of
the water, and these, at regular intervals, have a layer of silex
placed at the bottom. This silex is a plant like the rosemary in
appearance, rough and prickly, and well adapted for arresting any
pieces of gold that may be carried along. The sides, too, are closed in
with planks, and are supported by arches when carried over steep and
precipitous spots. The earth, carried onwards by the stream, arrives
at the sea at last, and thus is the shattered mountain washed
away—causes which have greatly tended to extend the shores of Spain by
these encroachments on the deep."
DEEP-MINING.
The two principal methods of Deep-Mining are Drifting and Hydraulicking.
Drifting.—Gold
is often mined in deep deposits by means of tunnels and drifts. This is
styled drift-mining, which, as a rule, is resorted to only in those
districts where the deposits are covered by an overflow from volcanic
sources, though in many instances the bottom stratum (sometimes
intermediate strata) has been drifted out of banks not capped with lava.
