ORIGIN OF THE DIAMOND 387
into
the original mass and remained, appears to have been segregated as
diamond, though a considerable amount escaped probably as carbon
dioxide.
The
inference that the supply of carbon came from the surface seems
justified also by the fact that the yield of diamonds in the chimneys
was greatest in the upper levels. It is true that the surface yield of
some of them was less than at a depth of several hundred feet, and that
in one or two cases where the yield has been small from the beginning,
the percentage continues very even, but generally there is a steady
decline in the percentage of yield as the workings are carried to
greater depths.
The
sinking of the earth's crust outside the borders of the diamond plateau
and the natural gravitation of the plateau itself, would establish a
steady pressure upon the underlying molten material and force the magma
up the vertical fissures and into and through the surface material
draining into them. This would result in the heating of the surface
supply, the cooling of the magma, and the amalgamation of both. The
pressure, however, would not be constant. Occasional slips in the
readjustment of the earth's crust would suddenly force columns of the
cooling agglomerate upward, and this raising process would be repeated
until the mass had become sufficiently solidified to resist the
pressure from below. In this manner it is conceivable that the dykes
could have been filled as we find them, by successive upheavals of
separate columns.
Reviewing
facts and inferences that may be fairly drawn from them, it seems
probable that the diamond plateau of South Africa was left at an
elevation by the shrinkage of the earth's crust surrounding it.
