FORMATION OF THE DIAMOND
The Diamond-bearing Deposits
the basin now extending as an arid karroo for hundreds of miles to the
south of the Kim-berley Diamond Fields the waters of a great lake once
spread. It is apparent that the diamond mines are on the northerly rim
of this basin, for the beds of shale that everywhere underlie the
basaltic trap surface or country rock are notably thinner in the
northern mine openings than they are farther south at Bultfontein and
Dutoitspan,1 and shortly after passing Kim-berley fields the
shale terminates at the edge of the " bed rock " of the Vaal River
diggings, an amvgdaloidal trap which Dr. Stelzner'2 determined to be olivine diabase.
the great open excavations and the extension of the underground
workings, the rock formations of the karroo basin are very clearly
revealed. The red soil that covers the surface of the country to the
depth of from one to five feet is evidently the result of the
decomposition of the friable face of the underlying basalt, which is
scattered in fragments over the country in jutting boulders and rounded
stones. This rock at De Beers and Kimberley mines is from twenty to
ninety feet in thickness, but very much decomposed throughout. Below
the layer is a bed of black shale, ranging in thickness from two
hundred to three
1 "Diamonds and Gold in South Africa," p. 19, Theodore Reunert, M.E.
2 Dr. A. W. Stelzner, Professor of Geology at the Freiberg Mining Academy.