THE MATRIX OF THE DIAMOND 51
(1) Kimherlite proper, a typical porphyrinic lava ; (2) Kimberlite breccia, the same lava broken and crushed by volcanic movements and crowded with included fragments of shale ; (3) Kimberlite tuff, being
the fragmental and tufaceous portion of the same volcanic rock. These
varieties pass by insensible gradations one into another, so that no
sharp line can be drawn between them, and all occur together in the
same neck or crater. The Kimberlite breccia forms by far the greater
portion of the rock, and is rich in diamonds. It is traversed by dykes
of Kimberlite proper, and contains streaks and patches of softer '
soapy ' material, which appears to be the Kimberlite tuff. The deeper
portions of the Kimberlite breccia become more compact as though
passing gradually into Kimberlite proper. The so-called 'blue ground'
has also been analysed, but being so thoroughly altered, the analyses
are of small scientific value.
Perhaps
the most interesting chemical observation concerning this 'blue
ground' was that made by Sir H. E. Eoscoe. He found' that on treating
it with hot water an aromatic hydrocarbon could be extracted. By
digesting the ' blue ground ' with ether, and allowing the solution to
evaporate, this hydrocarbon was separated and found to be crystalline,
strongly aromatic, volatile, burning with a smoky flame, and melting at
50'C.
Structure.—The
structure of Kimberlite, to which subject we may now proceed, is its
distinguishing and characteristic feature, differentiating it from
ordinary serpentines and peridotites, if not from all other terrestrial
rocks. The structure can only be compared with that of certain
meteorites.
It
may be described as a porphyritic brecciated structure. While
evidently the structure of an eruptive rock, it at the same time shows
clear proof of mechanical disturbances during or after its cooling.
According as the por-
1 Manchester, Lit. Phil. Soc. Proc. xxiv. 1885, p. 5.
