In
other words, the roof received as much deposition as the floor. This
points to a considerable degree of surface tension between the solution
of silica and the pre-formed portion of the Agate; hut if the layer of
clear Chalcedony became thick, gravity in some cases overcame the
surface tension,and the newly deposited jelly-like silica sagged
somewhat, and as further layers were added the sagging would become
more marked, until a diminutive stalactite was formed. The presence of
such a stalactite increased the surface tension at that point, and
caused a greater relative deposition of new silica. In most cases small
growths of zeolitic minerals occurred on the " skin," often in the form
of minute sheaves of crystals; thus at such points the surface area was
much increased, and hence also the absolute surface tension, leading to
an extra deposit of silica there in layers concentric to these little
sheaves. In this manner were formed the " eyes " so commonly seen in
Agates. Where the eyes were close together the bands took on the form
of salient and re-entering angles, giving on cross-section a plan
somewhat similar to the plan of a fort; hence such Agates are called
Fortification Agates.
Should
much zeolitic matter be deposited with the Chalcedony the Agate has an
opaque chalky appearance and is then known as Chalcedony Cachalong.
Often too the silica is in the hydrous colloid form, Opal; or, again,
the last formed layers (in the centre) may be anhydrous and they then
take the form of Quartz, Amethyst or Cairngorm.
Now
Opal in its solution seems to have a much weaker surface tension, and
so, instead of finding it evenly coating the inside of the cavity we
see that it is deposited in horizontal layers; when the layers are
parallel Onyx is formed, but
