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Sec. II, Ch. 2: The African Diamond

Sec. II, Ch. 2: The African Diamond Page of 366 Sec. II, Ch. 2: The African Diamond Text size:minus plus Restore normal size   Mail page  Print this page
African Diamonds.
85
claim they are but sparsely scattered through the rock. Microscopic crystals of Diamond are disseminated through the blue earth. Each matrix is said to yield Diamonds easily distinguished from those of other pipes, so that buyers on the field can generally tell, on looking at a stone, from which locality it has been obtained. These local peculiar­ities suggest that the stones have been formed in or near the centres where they are now found, though probably at great depths. In support of this view, it has been pointed out that most of the crystals are sharp at the edges, and exhibit no signs of abrasion, such as we might expect to find had they been transported far from their original site ; but on the other hand, a large proportion of the crystals have evidently been shattered, and exist now as mere frag-ments,showing that the rocks have suffered great disturbance, probably during their projection to the surface from some deep-seated source.
It is interesting to note the nature of the rocks through which the volcanic material must have forced its way up­wards. Beneath the red soil of the country is a decom­posed basalt, and this is followed by black carbonaceous shales, dipping slightly to the north. The shales are from 200 to 250 feet in thickness, and it was suggested by the late Prof. Carvill Lewis, that the Diamonds may have resulted from the action of the olivine rock on the carbon of these shales. Beneath the shale is a bed of conglomerate, which rests upon an amygdaloidal olivine-diabase, often described as a melaphyre, and representing an old lava-flow, about 400 feet thick. The rock beneath this ancient lava is a quartzite of great but undetermined thickness.
Igneous dykes penetrate these rocks almost vertically. One of the most interesting of these dykes is the large mass in De Beers' mine, known as the " Snake." According
Sec. II, Ch. 2: The African Diamond Page of 366 Sec. II, Ch. 2: The African Diamond
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