DIFFERENT METHODS OF MINING. 83
Drifting
presupposes the concentration of the precious metal in a well defined
stratum or channel. This method has been extensively employed in many
parts of California, particularly in Placer, Sierra, and Plumas
counties.
Where
a pay channel has been found, or is surmised to exist, a tunnel is
driven to develop it. This tunnel must be run in such a manner as to
drain all parts of the mine, and its location is therefore a matter of
the greatest importance. Before commencing such a work, which may
require years for its completion and cost large sums of money, every
precaution should be taken to ascertain the exact position of the
channel. Want of knowledge on this point has caused disastrous
failures in but too many cases.
As
the channel can often be found only by means of tunnels, the risk of
undertaking drift-mining is apparent. In those fortunate instances in
which the channel is disclosed on the surface and rises as it enters
the hill, the tunnel is run along its bed, partially in the bed-rock.
Otherwise the tunnel is driven below the channel or through the
rim-rock, being located with such a grade that the deepest part of the
workings will be above it.
In
some claims shafts have been sunk and the gravel drifted out has been
raised through these shafts to the surface. This method is quite common
in Australia, but comparatively rare in California.
When
a tunnel has been properly located and the channel opened, drifts are
run through the pay ground on both sides and the material is breasted
out regularly, timbering being employed as the work may require.
Shafts must sometimes be raised to the surface for the sake of
ventilation.
The
gravel is removed through the tunnel by means of a tramway and carried
to the mouth, where it is dumped on floors and then washed in the
sluices. When too firmly cemented to be broken up by sluicing, the
gravel is crushed under stamps.
