some
250 feet in thickness ; the black colour is due in part to carbonaceous
matter and in part to disseminated Pyrites, as is the case in many of
the black shales of carboniferous age in England. Below this again is a
layer of somewhat decomposed basic volcanic rock containing amygdules,
of 394 feet. Lower still comes a 400 foot bed of quartzite, and under
that is another 260 feet of black shales; both the quartzite and the
black shales are penetrated by dolerite dykes. The general disposition
of these rocks is horizontal, or nearly so, except near the pipe, where
a dip away from the vent is sometimes seen. Below the lower shales
there is over 1,000 feet of quartz-porphyry in the Kimberley mine. The
Diamond-bearing ground consists of a mass of irregular fragments of
rock of various shapes and sizes, usually with more or less angular
outline, and consisting not only of the shales and basic volcanic rock
from the surrounding Karoo beds, but also of rocks not seen in the
neighbourhood; some blocks belonging to the Middle Karoo sandstones,
blocks of ultrabasic rock called eclogite, masses of mica schist, and
pieces of granite. Some of these masses may contain many thousand cubic
yards of material, others are minute splinters, and every gradation is
found between these extremes. All these fragments are embedded in a
bluish-green material consisting largely of a hydrous magnesium-iron
silicate, with some carbonate of calcium. The whole mass has a general
bluish tinge, except near the surface, due to the dissemination of this
material, and from this appearance it has been named "blue-ground."
Where this has been exposed to the weathering influence of surface
water, that is for a depth of 50 or 60 feet, the process of oxidation
has converted some of the ferrous compounds
