tion
by De Beers Consolidated Mines Limited enabled its directors to
institute and conduct successfully a single broadly comprehensive plan
for extracting the diamond-bearing rock and for disposing to the best
advantage the total product of their mines.
This
system' of mining was devised and applied by me shortly after my
appointment as general manager of the De Beers Consolidated Mines
Limited, and was based essentially on a method suggested by the miners
themselves and without reference to any other system. Instead of
attempting to withstand, even for a time, the pressure of the
superincumbent mass of broken reef, the new system contemplated the
caving in and filling of the excavations, after the precious blue
ground had been extracted.
In
order to make the output of diamond-bearing ground as great as
possible, the levels in De Beers mine were at first opened up in the
new system according to the following plan :—•
When the numerous small tunnels had been driven to the margin of the mine, i.e. to
the point where they reached the sides of the crater, the blue ground
was stoped on both sides of and above each tunnel until a chamber was
formed extending along the face of the rock for ioo or more feet, with
an average width of about 20 feet, and about 20 feet high. The roof of
the chamber or gallery was then blasted down or allowed to break down
by the pressure of the overlying mass of broken diamond-bearing ground
or debris. I mention diamond-bearing ground here, for in the early
stages of underground mining there was an enormous amount of this
ground which had been left behind when open mining was discontinued,
and had been crushed either by the moving sides of the immense opening
or by the collapse of the underground pillars when mined by the old
system. It happened frequently, after breaking through to the loose
ground above, that clean diamond-bearing ground would run down as fast
as it was removed for weeks or months at a time. The galleries would at
times become blocked with large pieces of blue ground, which had to be
blasted, and then a
