132 THE DIAMOND MINES OF SOUTH AFRICA
bonaceous
shales into cavities of the amygdaloidal rock which lies outside of the
volcanic pipes. Then he reduces this gas by enormous pressure to a
liquid state, and, having gotten it into a form, as he thought,
admitting of crystallization, he absorbs the oxygen of the carbonic
acid by the iron in the containing walls of the craters. Now, as a
matter of fact, there are no cavities in the amygdaloidal rock
underlying the shales, for all interstices are filled with silica in
the form of agates, or with calcite. Furthermore, if carbonic acid had
been left in the olivine diabase to crystallize, then the resultant
diamonds would have been enclosed in this formation, which is also
contrary to fact, for no diamonds have ever been found in the
amygdaloidal rock. His main contention, too, is the derival from the
shales of the carbon necessary for the formation of diamonds. It will
be made clear, subsequently in this discussion, that this assumption
is not justified.
The
late Henry Carvill Lewis, M.A., F.G.S., Professor of Mineralogy in the
Academy of Natural Sciences, Philadelphia, U.S.A., advanced the
proposition that the diamond is the result of the intrusion of igneous
rocks into and through the carbonaceous shales, and the
crystallization of the carbon throughout the rocks, as it cools, from
hydrocarbon, distilled from the shales that had been broken through.1
In
support of such a theory, it is claimed that the diamonds in the
various mines or pipes have different characteristics. It is quite true
that large parcels of diamonds from the various mines have distinctive
characteristics, and it can be easily told from which mine a parcel of
diamonds comes ; but it is very difficult to tell in which mine a
single stone may have been found, though each mine has stones in a
great measure peculiar to itself. Some observers claim that the broken
diamonds which are extracted are broken during the process of winning
them. It is admitted that diamonds may be broken in the process of
mining and the subsequent operations of winning, but these cases are
exceptional. Fragments of diamonds are very frequently found embedded
in the blue ground, and there 1 "The Matrix of the Diamond," Professor Henry Carvill Lewis.
