Ch. 16: Formation of the Diamond

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	CHAPTER XVI FORMATION OF THE DIAMOND The Diamond-bearing Deposits VER 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. By 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. 119

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