SECTION II
THE MATRIX OF THE DIAMOND
By H, CARVILL LEWIS, M.A., F.O.S. (Read at the Meeting of the British Association at Manchester, 1887)
At the
last Meeting of this Association ' I had the honour of giving a short
description of the remarkable rock which forms the matrix of the
diamond in South Africa. Since then, as I have received fresh material,
it has been possible to study it carefully, both microscopically and
chemically, and to compare the geological features of Kimberley with
those of other diamond localities in various parts of the world.
Without
repeating what was then said, I will merely remind you that the
diamond-bearing rock was shown to be an eruptive neck of post-Triassic
age, penetrating and enclosing fragments of Karoo shales, and that this
rock is a porphyritic peridotite of peculiar structure, closely
analogous to a similar rock in Elliott Co., Kentucky.2
The
rock, which was obtained from a depth of about 500 feet, is much less
decomposed than the material usually obtained in the diamond mines, and
both its composition and structure can be readily studied under the
microscope.
It
is a dark-green heavy rock, resembling a dense serpentine, in which one
sees with the naked eye glistening plates of brown biotite, small
deep-red garnets, and large davk-green crystals or grains of olivine
and bronzite.
