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ORIGIN OF THE DIAMOND 375
The
diamond chimneys of Africa are huge dykes or chasms, penetrating
vertically the strata of the country known as the Karoo formation, to
unknown depths, and filled evidently from below with a material quite
unlike any of the strata which wall the pipes. These walls are the
edges of the horizontal layers which form the crust of the earth in
that section. In the Kimberley district, under a varying surface
deposit of several feet of red clay and an underlying bed of calcareous
tufa 5 to 20 feet thick, which covers the pipes and the surrounding
strata alike, the layers consist of about 50 feet of pale shales of a
grayish color, under which is about 275 feet of black bituminous
shales. Beneath this are several hundred feet of melaphyr, about the
same thickness as the black shale, and under that, is quartzite and
olivine-rock. There are slight variations from this order owing to
faults and intrusions, as for instance in the strata about the
Dutoitspan mine, in which case there is a layer of quartzite above the
melaphyr and 63 feet of diorite between them, but shale, melaphyr,
quartzite, and granite or gneiss, is the usual arrangement of the Karoo
formation.
The
contents of these chimneys are in all cases similar. There are some
small variations of little importance, but the general character of the
contents of all the chimneys is the same, and in each chimney, except
for an alteration of color and consistency in the upper part of the
material filling it, due to weathering, the contents are precisely the
same as far as they have been followed down. The " blue ground," as the
diamondiferous material is called, at the 2,500 foot levels, is the
same as that 1,000 feet down, and both are the same as the yel-
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