Abstract
DIAMONDS were discovered in gravels of the Orange River in 1867, and were traced three years later to a peculiar earthy material called from its colour “yellow ground” by the miners. This, which was soon found to pass down into a more solid and dark-coloured material called “blue ground,” occupies “pipes” in the country rock—carbonaceous shales and grits belonging to the Karoo system; the one standing in much the same relation to the other as do the volcanic necks to the carboniferous strata in Fifeshire. Flows or sills of basaltic rocks are associated with the sedimentary strata, and both are cut by dykes. The matrix of the blue ground is a fine granular mixture, chiefly consisting of a carbonate (calcite or dolomite) and serpentine. In this are embedded grains of garnet (mostly pyrope), pyroxenes (a chrome diopside, smaragdite or enstatite), a brown mica, magnetite and other ores of iron, and some other minerals more sparsely distributed. Rock fragments also occur; some of them are the ordinary shale and grit, but others are compact and of an uncertain aspect. Crystalline rocks are sometimes found.
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BONNEY, T. The Parent-Rock of the South African Diamond1. Nature 60, 620–621 (1899). https://doi.org/10.1038/060620a0
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DOI: https://doi.org/10.1038/060620a0