50 THE MATRIX OF THE DIAMOND
and chassignitel (an
olivine meteorite). The structure of the Kimberley rock, as will
presently be shown, differentiates it from these, which are all
holocrystalline. The rock now described appears to differ from any
heretofore known. Picrite-porphyrite is practically a felspar-free
melaphyre, or, perhaps in deference to its hornblende, it may be called
the effusive form of the rock named camp-tonite by Rosenbusch.
Picrite-porphyrite is also an augitic rock, and contains less magnesia
than the Kimberley rock. Limburgite, the neovolcanic equivalent of
picrite-porphyrite, is likewise an essentially augitic rock, being a
non-felspathic basalt,2 and is also much poorer than the Kimberley rock.
There
appears to be no named rock-type having at once the composition and
structure of the Kimberley rock. For this reason, as also on account of
its importance as the matrix of the diamond, it is now proposed to name
the rock Kimberlite.
Kimberlite
may be described as a porphyria volcanic peridotite of basaltic
structure, or, according to Rosen-busch's nomenclature, the
palaeovolcanic ' ergussform ' of a biotite-bronzite-dunite, being an
olivine-bronzite-picrite-porphyrite rich in biotite. Had it less
olivine and more rhombic pyroxene it could be classed among the
picrite-porphyrites, and be called a saxonite-porphyrite. As it is, it
is more nearly related to a dunite-porphyrite. Prof. Judd has described
a porphyritic dunite, but that is a holocrystalline deep-seated rock
of entirely different structure from the rock under consideration. In
fact Kimberlite is a rock mi generis, dissimilar to any other known species.
Three varieties of Kimberlite may be distinguished:
