ORIGIN OF THE DIAMOND 381
sharp
angle, and occasionally set up vertically, but in South Africa the
strata lie in their natural horizontal position, undisturbed apparently
except for these vertical dykes which make a clean boring through the
even, natural formation. Igneous intrusions exist in places among the
strata, and a stratum of basalt caps the shale about some of the mines,
but they are quite independent of the diamondiferous contents of the
chimneys, and do not appear to have had any influence upon the
kimber-lite, or to have been acted upon by it.
It
is evident that these chimneys are not the vents of sudden, local,
igneous, volcanic, eruption. Not only is the crater formation absent,
but there has been no overflow nor scattering of ashes or lava about
the mouth of any one of them. The contents have apparently been raised
to, or a little above, the surface of the surrounding land by a series
of uplifts, or forced upward by the subsidence of the entire plateau.
Nor do the edges of the surrounding strata forming the walls of the
chimneys, show any sign of igneous action. The face of the quartzite
stratum is even and unaltered; the highly inflammable black shale,
though bent upwards at the edges as if by pressure from below and the
expansion of the contents of the chimney, carry no signs of firing, and
the horizontal trend of the strata is undisturbed. The composition of
the kimberlite breccia also suggests the idea that it was not
solidified from a molten condition. It contains large quantities of the
black shale, and the diamonds are said to be most plentiful where the
shale inclusions are most abundant.
Nevertheless, Henry Carvill Lewis, in " The Matrix of the Diamond," edited by Prof. T. G. Bonney, says:
