Below the conglomerate is a very hard amygdaloidal rock,called melaphyre by M. A. Moulle,1 which was finally determined by Dr. Stelzner'2
to be olivine diabase. Its mineral composition is the same as
melaphyre,—plagioclase, augite, and olivine, but one is granular and
the other porphyritic. It is about four hundred feet in thickness and
is very hard. Underlying the melaphyre is quartzite, about seven
hundred feet thick, with quartz porphyry below it, the thickness of
which is undetermined. The Kim-berley rock shaft has passed through one
thousand feet of it, and the bottom of the shaft is still in the same
formation. All these strata lie nearly horizontal, but dip slightly to
the southeast. They are graphically presented in the sectional views of
the rock shafts of the several mines shown on page 120.
Through
these layers of rock extend from an unknown depth the huge pipes
containing the diamond-bearing deposits, or blue ground, which is a
breccia filled with fragments of shale and other minerals. These
immense funnels are obviously extinct craters filled with volcanic mud
from below. All evidence to hand points to an aqueous formation, and
the upheaval is shown by the upturning of the enclosing shales at
various places in contact with the blue ground.8 Many
boulders are found in the blue ground of the same composition as the
surrounding rock, but others have undoubtedly come up from greater
depths than have yet been reached by the sinking of shafts. It is,
however, highly remarkable that there was almost no apparent overflow
in the filling of these craters, for the diamond-bearing ground is
either level with the surrounding surface, or rises, usually, only a
few feet above it in kopjes or hillocks. Outside of the mouths of the
craters no diamonds have been found except at Dutoitspan, where the
upheaval formed quite a hill, and some diamonds have been taken from
the surrounding ground within a few yards from the margin of
1
" Memoire sur la geologie generale et sur les mines de diamants de
l'Afrique du Sud." Annales des Mines, 8th Series, Vol. VII (1885), p. 193.
2 Dr. A. W. Stelzner, Professor of Geology at the Freiberg Mining Academy.
3 Still to be seen at De Beers Mine.
