components
of an impure limestone or other sedimentary rock may be rearranged so
that Zircon or Sphene may be formed, amongst many other minerals.
Under B2,
practically the only minerals which concern us are Fluor Spar and
Quartz; in a mineral vein both these may result from the cooling of
heated waters rising up through the great fault-fissures. It may be
noted that the sequence of deposition of these minerals is usually
constant in any given vein, and that such sequence can in most cases be
very easily determined from an examination of the vein.
The group B5
includes the great majority of the minerals used as gem stones. Their
origin appears to be very similar to that of the first group of
Hypogene minerals spoken of. They may be regarded as falling into two
sub-groups; firstly, those whose whole constituents existed in the rock
before its change ; and secondly, those whose formation demanded the
introduction of some fresh elements from without the rock. In both
cases the molecular rearrangement would appear to have been brought
about by the slow action of weakly alkaline water under considerable
heat and pressure. In the first sub-group a calcareous rock containing
the usual numerous impurities may, by molecular or atomic
reconstruction, have formed in it such minerals as Idocrase, Sphene,
Spinel and several of the Garnets, besides many other minerals which
concern us less. A rock of a clayey nature provides the materials for
the formation of Iolite, Andalusite, some of the Garnets, and other
compounds. One point of great interest is that Graphite or other forms
of carbon diffused in a rock may by the action of heated water be
converted into (possibly) a metallic carbide from
