Of
the shape of Agates it must suffice to say it follows that of the
containing steam cavity; if this cavity were formed in a stationary
viscid mass of rock it would have much the form of a bubble of air
slowly rising through glycerine; this common form may be called
balloon-shaped. Where the lava stream has been slowly moving at the
time the cavity was formed the cavity becomes drawn out just as our
imaginary air bubble would if the glycerine were slowly poured on a
cold day from one vessel to another : it is lenticular or
almond-shaped, and hence such cavities are called amydaloidal cavities.
Two cavities may coalesce just as solidification is proceeding, and in
this case an Agate having a compressed dumb-bell shape may be found. Or
again a slight faulting may occur after consolidation before the
filling of the cavity, resulting in the dislocation being apparent in
the Agate. All these points help in the field to determine an Agate
amongst a mass of other generally rounded stones. The size of the
cavities varies from a microscopic one to one of many feet in diameter.
The varieties of Agate have already been indicated for the most part, but it may be well to summarise them.
The
commonest form is the Banded, and this when the bands are nearly
circular is called the Ring Agate. If the bands are well marked, narrow
and fairly straight, it is a Eibbon Agate, though this term is
frequently applied to a certain kind of Vein Agate too. Onyx Agate owes
its character to the opaline bands causing a parallel arrangement of
the layers, the straight-banding being the characteristic point of
Onyx. Stalactitic Agates show stalactites hanging down from what was
the roof or dome of the cavity.
