[Illustration: _Fig. 12._--KIMBERLEY MINE, 1874.]
[Illustration: _Fig. 13._--KIMBERLEY MINE, 1902. (From Photographs by
C. Evans.)]
In a very few years, however, the open pit mining was rendered
impossible by the mud rushes, by the falls of the masses of barren
rock known as "reef," which were left standing in the mine, and by
landslips from the sides, so that in 1883, when the pit had reached a
depth of about 400 ft., mining in the Kimberley crater had become
almost impossible. By 1889, in the whole group of mines, Kimberley,
Dutoitspan, De Beers and Bultfontein, open pit working was practically
abandoned. Meanwhile mining below the bottom of the pits by means of
shafts and underground tunnels had been commenced; but the full
development of modern methods dates from the year 1889 when Cecil
Rhodes and Alfred Beit, who had already secured control of the De
Beers mine, acquired also the control of the Kimberley mine, and
shortly afterwards consolidated the entire group in the hands of the
De Beers Company. (See KIMBERLEY.)
The scene of native mining was now transferred from the open pit to
underground tunnels; the vast network of wire ropes (Plate II. fig.
12) with their ascending and descending buckets disappeared, and with
it the cosmopolitan crowd of busy miners working like ants at the
bottom of the pit. In place of all this, the visitor to Kimberley
encounters at the edge of the town only a huge crater, silent and
apparently deserted, with no visible sign of the great mining
operations which are conducted nearly half a mile below the surface.
The aspect of the Kimberley pit in 1906 is shown in fig. 13 of Plate
II., which may be compared with the section of fig. 8.
In fig. 13, Plate II., the sequence of the basalt, shale and melaphyre
is clearly visible on the sides of the pit; and fig. 8 shows how the
crater or "pipe" of blue ground has penetrated these rocks and also
the underlying quartzite. The workings at De Beers had extended into
the still more deeply seated granite in 1906. Figure 9, Plate I.,
shows the top of the De Beers' crater with basalt overlying the shale.
Figure 8 also explains the modern system of mining introduced by
Gardner Williams. A vertical shaft is sunk in the vicinity of the
mine, and from this horizontal tunnels are driven into the pipe at
different levels separated by intervals of 40 ft. Through the blue
ground itself on each level a series of parallel tunnels about 120 ft.
apart are driven to the opposite side of the pipe, and at right angles
to these, and 36 ft. apart, another series of tunnels. When the
tunnels reach the side of the mine they are opened upwards and
sideways so as to form a large chamber, and the overlying mass of blue
ground and debris is allowed to settle down and fill up the gallery.
On each level this process is carried somewhat farther back than on
the level below (fig. 8); material is thus continually withdrawn from
one side of the mine and extracted by means of the rock shaft on the
opposite side, while the superincumbent debris is continually sinking,
and is allowed to fall deeper on the side farthest from the shaft as
the blue ground is withdrawn from beneath it. In 1905 the main shaft
had been sunk to a depth of 2600 ft. at the Kimberley mine.
For the extraction and treatment of the blue ground the De Beers
Company in its great winding and washing plant employs labour-saving
machinery on a gigantic scale. The ground is transferred in trucks to
the shaft where it is automatically tipped into skips holding 96 cubic
ft. (six truck loads); these are rapidly hoisted to the surface, where
their contents are automatically dumped into side-tipping trucks, and
these in turn are drawn away in a continual procession by an endless
wire rope along the tram lines leading to the vast "distributing
floors." These are open tracts upon which the blue ground is spread
out and left exposed to sun and rain until it crumbles and
disintegrates, the process being hastened by harrowing with steam
ploughs; this may require a period of three or six months, or even a
year. The stock of blue ground on the floors at one time in 1905 was
nearly 4,500,000 loads. The disintegrated ground is then brought back
in the trucks and fed through perforated cylinders into the washing
pans; the hard blue which has resisted disintegration on the floors,
and the lumps which are too big to pass the cylindrical sieves, are
crushed before going to the pans. These are shallow cylindrical
troughs containing muddy water in which the diamonds and other heavy
minerals (concentrates) are swept to the rim by revolving toothed
arms, while the lighter stuff escapes near the centre of the pan. The
concentrates are then passed over sloping tables (pulsator) and shaken
to and fro under a stream of water which effects a second
concentration of the heaviest material.
Until recently the final separation of the diamond from the
concentrates was made by hand picking, but even this has now been
replaced by machinery, owing to the remarkable discovery that a
greased surface will hold a diamond while allowing the other heavy
minerals to pass over it. The concentrates are washed down a sloping
table of corrugated iron which is smeared with grease, and it is found
that practically all the diamonds adhere to the table, and the other
minerals are washed away. At the large and important Premier mine in
the Transvaal the Elmore process, used in British Columbia and in
Wales for the separation of metallic ores, has been also introduced.
In the Elmore process oil is employed to float off the materials which
adhere to it, while the other materials remain in the water, the oil
being separated from the water by centrifugal action. The other
minerals found in the concentrates are pebbles and fragments of
pyrope, zircon, cyanite, chrome-diopside, enstatite, a green pyroxene,
mica, ilmenite, magnetite, chromite, hornblende, olivine, barytes,
calcite and pyrites.
In all the S. African mines the diamonds are not only crystals of
various weights from fractions of a carat to 150 carats, but also
occur as microscopic crystals disseminated through the blue ground. In
spite of this, however, the average yield in the profitable mines is
only from 0.2 carat to 0.6 carat per load of 1600 lb., or on an
average about 1-1/2 grs. per ton. The annual output of diamonds from
the De Beers mines was valued in 1906 at nearly L5,000,000; the value
per carat ranging from about 35s. to 70s.
[Illustration: SECTION OF KIMBERLEY MINE LOOKING EAST