THE MATRIX OF THE DIAMOND 55
tion.
In this case the old lava forms the larger part of the breccia, and the
final composition and structure is not unlike that of a tuff. It is of
course just this material which, when ejected in the form of dust or
mud, does make the stratified ashes called tuff. This kind of breccia
occurs in the Euganean Hills,1 in Italy,'2 and perhaps in Wales.3
According to Chaper,4
who has studied the geology of the Kimberley mines, the eruption has
taken place at many successive periods; all his observations pointing
to a series of eruptions, between each of which there was time for the
volcanic mass to consolidate. He says that the great irregularities so
puzzling to the miners are thus explained, and that at Bulfontein and
Du Toits Pan it is possible to make out a chronology of the various
eruptions.
If
this last is the principal cause of the breccia it is no wonder that
the rock should so closely resemble a tuff. It is, of course, difncult
to draw the line between a brecciated lava and a tuff, but in this
instance everything points to its lava-like character. The tuff would
lie outside of the volcano, the brecciated lava in its vent. The
conception of a true porphyritic lava, afterwards broken by paroxysmal
eruptions, seems perfectly to explain the nature of the remarkable rock
which is now under discussion. Certain so-called ' soapy' bands in the
mine probably represent the true tuff. Just as the limburgite of the
Kaiserstuhl district alternates with beds of tuff, so here it is
probable that
