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PRECIOUS STONES.
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which
a pure form of carbon may be deposited in visible quantity. Anthracite
probably thus formed may be found in many agglomerates, and it was
suggested by J. G. Good-child that Diamond may arise in this way;
certainly Diamonds are found in a similar rock (agglomerate) in the
great mines of South Africa.
Rocks
of eruptive origin have developed in them minerals bearing naturally a
relation in composition to tbat of the parent rock, so that one of
basic or sub-basic character may have Epidote and Garnets formed in it.
Of
the second sub-group Tourmaline is a good example, though this mineral
usually shows evidence of dynamic action as well. Topaz, Beryl,
Euclase, Phenakite and Chrysoberyl very probably have some of their
rare conÂstituents brought to the rock in this way. Topaz and Beryl
often occur in the druses of plutonic rocks (as granite) which seem to
have been formed by the liberation of aqueous vapour at the outer parts
of the granite mass at a later stage in its consolidation; these
cavities often have several minerals deposited in them in a regular
succession, Quartz (often as Cairngorm) being among the earlier, Topaz
and Beryl among the later. In such druses, where the crystals only form
a lining, the crystal forms are idio-morphic, but where the crystals
have grown so as to touch, they are allotriomorphic, and in some cases
such growth is associated with absorption of the substance of the
crystal to some extent ; in these instances the absorption is found to
occur almost entirely along the axis of greatest elongation.
Tourmaline presents a good picture of the changes that occur in the formation of itself and other minerals which
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