2 ON A DIAMOND-BEARING PERIDOTITE AND
of
which could be enclosed by a circle 3-1/2 miles in diameter. The mines
lie at the northern end of a great plateau, known as the Upper Karoo
Plateau, which extends from the Bokkeveldt mountains at the Cape of
Good Hope to the border of the Transvaal Republic, varying in elevation
from 2,700 to 6,000 feet above sea level.' The four principal mines at
Kimberley, at an elevation of about 3,900 feet, are close to one
another. The two principal mines in the Orange Free State
(Koffee-fontein and Jagers-fontein) lie S.E. of Kimberley, the former
30 miles, the latter GO miles distant.
In
1872, 30,000 persons had assembled about the four mines, and other
mines were soon discovered in the neighbouring territory. At the
present time, 15 distinct diamond mines (dry diggings) are known in
Griqua Land, Urot and the adjoining Orange Free State ; none of the
others, however, as yet equal in richness the four great mines first
discovered. These four all have the same geological structure, each
being a separate pipe; and all are remarkably rich in diamonds. It has
been estimated that since the opening of these mines more than six tons of diamonds 2 have
been extracted from them, being probably greater than the total
combined previous production of all the other mines in the world. It
was soon discovered that these pipes went down vertically to an unknown
depth, penetrating the surrounding strata.
The
diamond-bearing material first excavated was a soft yellowish friable
substance, readily crumbling when exposed. At the depth of about 100
feet it became darker and harder, and finally acquired a slate blue or
dark green colour, resembling some varieties of serpentine. This is the
well-known ' blue earth ' of the diamond mines, which proved to be
richer in diamonds than the wholly decom-
1 Cape of Good Hope Official Handbook, 188G, p. 109.
2 T. Keunert, Cape of Good Hope Official Handbook, 1886, p. 212. Moulle, ' Mimoire sur la gt'ologie generate et sur les mines de dianiants de l'Afrique du Sud,' Ann. des Mines, vii. 1885, p. l93.
