Matrix of the Diamond | Matrix Diamonds Page 22



	10	THE MATRIX OF THE DIAMOND

	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.