the
terrn refers to the parallelism of the bands and not to their
composition, hence Onyx may consist of Chalcedony. Opal containing
finely divided zeolitic material is true Cachalong.
That
all these layers remained in the form of a jelly until (in most cases,
at any rate) the cavity was entirely filled, is clearly shown in many
Agates by what Dr. Heddle called the " tube of escape." The contents
being denser, the osmotic pressure was greater; possibly also heat was
liberated on the crystallisation of some of the silica ; at any rate,
the weakest part of the skin gave way and a minute pore was formed
through which some of the silica jelly was forced, the gelatinous
layers of silica near the opening being at the same time bent outwards.
All
such true Agates being formed in closed cavities assume the shape of
these spaces, which, since they are often steam cavities in lavas, are
in many cases lenticular, similar in form to the bubbles in inferior
glass. Thus one of the means of recognising an Agate before it is cut
into is its shape.
When
ferric oxide is deposited in the Chalcedony in minute particles it
imparts a translucent blood-red or flesh colour to the Agate, which is
then called Carnelian.
Some colloid substances are liable to a molecular change causing them to pass into a crystalline form (cf. Roman
glass). Opal is so liable, and the change is accompanied by decrease in
volume, hence cracks are formed ; these cracks are sometimes penetrated
by solutions of hydrous oxide of manganese, which is deposited in
dendritic forms : such Agates are called Mochas.
Vein Agates are formed in fissures or spaces in communi-
