from
lower strata. Such a mass of volcanic material is called an
agglomerate, and, as is not infrequently the case in other
agglomerates, it shows signs of more than one outburst of volcanic
activity, for the whole mass is sub-divided in several of the mines
into vertically columnar portions which seem to mark a later eruption,
which has resulted in the material already choking the vent being
broken through and a fresh vent being formed, to be itself afterwards
choked up. Much of the matrix material must originally have consisted
of a ferro-magnesian silicate by the hydro-metamorphism or
thermo-metamorpbism of which the serpentinous minerals have originated.
It is entirely in this blue ground that the Diamonds occur, none ever
having been found in the surrounding rock.
In
the De Beers mine a dyke some five feet thick traverses the blue ground
in a sinuous path, and is hence known as the " snake rock." It is of
much the same composition as the blue ground, but contains no
Diamonds. In the South African Museum there is a specimen of dyke from
the blue ground of the De Beers mine, which shows good crystals of
Olivine in a ground mass of Perofskite, Magnetite, and Calcite. The
occurrence of Diamond in agglomerate near Pretoria is of interest, as
here the geological horizon is far below the carbonaceous shales
(Molengraaff).
The
associated minerals in the Kimberley district consist of a large
variety of species, chief among which in point of abundance are Garnet,
Enstatite, and altered Mica. The Garnet is of the variety Pyrope, and
occurs in broken fragments, often fresh, but sometimes partly
decomposed Chromium is always present in it. The Enstatite is often
found enclosing Garnet. Amongst other minerals of less
