122 PRECIOUS STONES.
On
reviewing the whole of these occurrences, it will be noticed that in
the great majority of cases the Diamond is found in material derived
from the wasting of older rocks. In some cases this wasting is
undoubtedly the result of ordinary weathering by the chemical and
physical effects of water in decomposing and mechanically wearing away
the rocks. In other cases it is more than likely it was due to the
influence of great alternations of heat and cold in the atmosphere,
principally aided by wind and blown sand. The resulting detritus may
occur as a loose sand or gravel, or may be re-cemented into a more or
less compact sandstone or conglomerate. In such cases we can learn
little of the original rock in which the Diamond was developed except
by a study of the associated minerals. Where these occur in a fresh
condition we may reasonably assume they fairly represent the original
constituents of the rock from which they came ; but in all cases it
must be borne in mind that the Diamond, by its hardness and chemical
stability, might have, so to speak, outlived all its companions, and
later have become associated with a fresh group of minerals. In such a
case we should expect tbe associated minerals to be different in many
instances ; but a review of the whole associates shows a remarkable
similarity in their kind, so we may here tabulate the principal
associated minerals mentioned in the foregoing pages:—
Vaal River.—Garnet, Ilmenite, Vaalite (a hydrous ferro-magnesian silicate).
Kimberley.—Garnet,
Enstatite, Biotite, Ilmenite, Magnetite, Bronzite, Chrome Diopside,
Smaragdite, Tre-molite, Asbestos, Zircon, Cyanite, Sapphire, Topaz,
Eutile, Tourmaline, Wollastonite, Serpentine.
