low
ground which was found near the surface, except that exposure to the
weather there had oxidized and turned it to a yellow color, instead of
the greenish-blue it is below.
In
most cases, the upper part of the contents of these chimneys formed
small rounded hills or kopjes, ten or more feet above the surrounding
level. The filling of the Wesselton only showed a depression. The
dia-mondiferous material of the chimneys is quite unlike the
surrounding reef. Without affecting the surrounding strata in any way,
it usually fills the dykes to the walls, though there are intervals, in
places, between the walls and the contents, and in these hollows are
numerous calcite crystals. Nor do the walls show any signs of abrasion
or heat, though the edges of the shales were bent upward slightly, as
if by pressure from below.
The
diamondiferous rock is a greenish-blue mineral, like dried mud with
numerous inclusions. It carries many fragments of the surrounding reef,
pieces of the shales being very noticeable. These foreign inclusions
vary in size from very small pieces to one so large that it is called "
the island." This is a block of olivine-basalt in the De Beers mine,
having an area of nearly 3,000 square feet and penetrating to a great
depth. Some inclusions must have been brought up from great depths, as
they differ from any of the strata which compose the reef. Large
blocks of gray sandstone, found at a depth of 250 feet, resemble the
sandstone which in other localities forms part of the middle Karoo
formation, and may be here an underlying stratum at great depth. These
foreign inclusions, differing entirely in nature from the
diamondiferous material with which
