G ON A DIAMOND-BEABING PEBIDOTITE AND
Additional particulars, of which a brief summary is subjoined, are supplied in more recent papers by Mr. Dunn,1 which are founded upon facts brought to light by fresh excavations.
The
bedding of the black shales surrounding the mines is turned upwards at
the edges of the pipe. These shales are very combustible and
carbonaceous; in one part of Kimberley Mine, where accidentally fired,
they have smouldered on for eighteen months. The shales extend at least
forty miles away, underlying the whole district. Diamonds are most
abundant where the pipe is surrounded by shales. The author suggests
that the carbon for the diamonds was supplied by these shales. If so,
the atmosphere would be the original source of the diamond, for the
plants absorbed carbonic acid from the air, and their remains made the
shales carbonaceous.
These
shales belong to the Karoo beds. In Camdebro anthracite occurs in these
beds; perhaps the result of distillation, due to a large dyke which
underlies the anthracite.
The dyke-like masses2
at De Beers differ from the main mass only in being finer grained and
less readily decomposed. They are two to three feet thick, and cut
through the pipe and the shales and dolerite in all directions. Mr.
Dunn also shows that the pipes must be more recent than the dolerite
sheets, for the rudely tabular dolerite is tilted up at 40° at the wall
of the pipe. Included masses of dolerite also occur in the pipes,
which at Bulfontein have been rounded by attrition into boulderlike
masses.
Among recent papers on the diamond regions must be
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