phane,
Cyanite, green Augite, Quartz, Corundum, Rutile, Magnetite, Staurolite,
Andalusite, Tourmaline, Epidote, Oligoclase; there is here a strong
presumption that the Diamond is from the pegmatite bands in the gneiss.
Omai Creek.—Spinel, Ilmenite, Corundum, Tourmaline, Topaz.
Some
of the minerals yield us little information from the mere fact of their
occurrence in association with Diamond. Quartz and the other varieties
of silica, for instance, occur under such diverse conditions that we
can here draw no conclusions as to the conditions under which Diamond
was developed.
Many
of the others, however, are found to occur under very similar
geological conditions in different localities. Thus Tourmaline, Eutile
and Zircon are usually found in rocks which have been subjected to
marked deformation; they are minerals of dynamo-metamorphic origin, in
other words. Beryl, too, often so results, but it and Topaz are
sometimes found to have crystallised out late in the consolidation of
granite masses. The oxides of iron, titanium, and chronium form an
interesting group with many interrelations. Many basic eruptive rocks
contain as an original constituent a slightly titaniferous oxide of
iron; their pyroxenes too often contain titanium, replacing silicon ;
the titaniferous oxide of iron occurring thus was designated Iserine to
distinguish it from ordinary Magnetite. When such a rock is subjected
to dynamo-metamorphism, the Iserine splits up, true Ilmenite resulting
in some cases, possibly with accretion of titanium from the pyroxenes.
At other times a titaniferous form of Haematite results, as is often
seen in the Haematite-schists such as have been
