all-important
factor, time. But in Nature's laboratory it is quite different, and the
rock-forming minerals of the lava are slowly decomposed, a small
portion of the material being carried deeper into the rock. Now, a lava
frequently contains steam holes, or vapour cavities, and if the
solution meet* one such in its downward path, it is carried into the
cavity by osmosis. Usually the first solution to enter is one derived
from the attack of the water on one of the ferro-magnesian minerals,
and once it is in the cavity a thin film of a mineral (in most cases
one allied to Serpentine) is deposited on the wall of the cavity, and
forms what is called the " priming." Dr. Heddle showed this priming to
consist in most cases of Celadonite. The thickness of this layer may
vary from a mere film to one entirely filling the cavity. If the
Celadonite continued to be deposited along with silica in the form of
Chalcedony, it often took the form of mosslike growths, surrounded by
the clear Chalcedony, giving rise thus to a Moss Agate. If the
Celadonite and Chalcedony were intimately mixed Prase or Heliotrope
would be formed. Later silica in solution was carried into the cavity
by osmosis, and deposited next inside the priming, or only separated
from the Celadonite by a thin layer, to be described immediately. The
deposition of the silica took the form of Chalcedony, and was very
probably brought about by the escape of the solvent. Often the
decomposition of the ferro-magnesian silicates was closely followed by
the disintegration of the felspars of the rock, and the resulting
products carried into the cavity and deposited immediately within the
Celadonite, as a layer of one of the Zeolites. Rarely the entire cavity
might be filled with Zeolite. All three of these minerals form coatings
of practically uniform thickness.
