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THE MATRIX OF THE DIAMOND 45
character of the rock, strong reasons for holding that the diamonds now lie in their original matrix.
Ground-mass.—All
the above-described minerals lie, some of them porphyritically, in a
ground-mass or base, which we may now examine. This is a more or less
homogeneous serpentinous mass, which, by reason of the decomposition it
has suffered, is very difficult to study. It is now mainly composed of
a nearly isotropic serpentinous mineral mixed with calcite. By treating
the slide with hot hydrochloric acid, the iron and calcite are
dissolved away and the amorphous and more readily soluble portions of
the ground-mass eliminated, so that traces of the original structure
for the first time become visible. Forms of small prismatic crystals
(possibly a pyroxenic mineral) can now be seen occasionally, and
between them are traces of an isotropic substance which may indicate a
non-crystalline base. Certain resemblances can be traced to the
ground-mass of sundry decomposed basaltic or other basal rocks. The
base now, however, is a greenish serpentinous substance full of calcite
grains, flour-like grains of gibbsite, rounded dusty masses of
ferruginous and kaolinic material, &c, the original structure being
entirely lost through disintegration. Until fresher specimens are
obtained it is impossible to speak with any confidence concerning the
original structure of the ground-mass. In one case already mentioned, a
brown glass was noticed enclosed in olivine. Any similar basic glass in
the ground-mass has long since been entirely decomposed.
Fragmented Enclosures.—Fragments,
both of the adjoining shale and diabase, and also of more deeply
seated granite, gneiss, eclogite and other rocks, occur in the
Kimberley peridotite. Those of shale are by far the most numerous,
sometimes becoming so abundant as to form the greater portion of the
rock.' A breccia of shale
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