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also
occur in rocks of other geological ages; perhaps they are even now
being formed in the earth's interior. Another interesting period is the
Cretaceous, i.e. a period only about 60 million years ago. In the
Cretaceous of Australia occur opals while in Colombia emeralds occur in
limestone. During this age the diamond pipes of South Africa were also
formed.
Rocks forming the. earth's crust
Rocks forming the earth's crust are grouped into three main divisions: (a) igneous,
(b) sedimentary, and (c) metamorphic according to their origin, without
reference to their geological age and representatives of each class may
occur in all geological ages and formations.
(a) Igneous rocks.—These
consist of silicate minerals crystallized from a molten magma. They are
of diverse kinds, varying in chemical composition, mineral
composition, structure and texture. The igneous rocks are either
extrusive or volcanic or intrusive or plutonic. The volcanic rocks have
cooled quickly under atmospheric pressure and may even be glassy like
obsidian or partly crystalline like basalt. Small crystals of hauyne,
sapphire or red zircon may occur in them. The igneous rocks have cooled
slowly under great pressure deep below the crust. They are coarsely
crystalline and their mineral constituents could be easily identified.
The following igneous rocks may contain some gem minerals:—
(a) granite may contain good crystals of beryl, topaz,
zircon, etc.
(b) Syenite „ „ corundum (ruby and
sapphire) and
zircon,
(c) Peridotite „ „ corundum.
These minerals are primary, but as they are present only in very small amounts they are termed accessory minerals;
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