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386 THE DIAMOND
notably
New York State and Kentucky, without any, and in Arkansas though some
diamonds have been found in a large body of it there, it is doubtful if
that contains any considerable quantity. It is evident, therefore, that
if the elements contained in kimberlite, under certain conditions, are
requisite for the crystallization of carbon, the presence of carbon
and its crystallization have nothing to do with the peculiar formation
of kimberlite. The South African chimneys are also traversed by dykes
of kimberlite which contain few if any diamonds. It is the breccia, or
more decomposed kimberlite containing the shale enclosures, which is
diamondiferous. This black shale in the stratum surrounding the
chimneys is combustible, but the fragments in the breccia have lost
their sulphur and carbonaceous matter. Few diamonds are found in purely
igneous or metamorphic rocks, though Henry Carvill Lewis referring to
kimberlite says, " Certain resemblances can be traced to the
ground-mass of sundry decomposed basaltic or other basal rocks." Sir H.
E. Roscoe found on treating " blue ground" with hot water, " an
aromatic hydrocarbon could be extracted, and by digesting it with ether
and allowing the solution to evaporate, this hydrocarbon was separated
and found to be crystalline, strongly aromatic, volatile, burning with
a smoky flame and melting at 50° C."
These
facts remind one again of the probable surface conditions existing at
the time of the opening of the diamond chimneys through the earth's
crust. It is noticeable too, that beyond the trace of hydrocarbon in
the ground-mass, and the carbon in the calcite, which is a decomposed
product, all the carbon which entered
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