After the crushing was done the cracks in the Carboniferous Limestone
were filled with water heavily charged with calcium carbonate, taken in
solution from the rock, first from pulverized particles, and afterwards
by percolation and contact with exposed surfaces. This calcium carbonate
was slowly deposited in crystalline form, so that in time the cracks
were filled and the crushed rock firmly cemented with calcite seams. But
in the meantime the removal of the calcium carbonate had started
disintegration of the more exposed portions of the rock, which steadily
continuing, finally reduced the porous body between the crystal seams to
a soft clay which was gradually dissolved and carried out through small
imperfections in the thin crystal sheets, leaving the empty box work as
we find it. But where blasting has exposed fresh surfaces, much of the
solid limestone carries the box-like sheets of crystal.
The thinnest box work is seen in the upper levels, from which the waters
retired soonest, and the heaviest and most beautiful is in the Blue
Grotto, on the eighth level where the water remained longest and its
diminished volume became most heavily charged. In many places, however,
there is another heavy variety known as pop-corn box work, which seems
to be an impure lime carbonate not so finely crystallized as the other,
but at the time of my visit no explanation had been given of the manner
of its deposit; and my own theory that it was not formed under water had
nothing to sustain it until, a few weeks later, while visiting Crystal
Cave, the work was found in active progress on surfaces occupying every
position, and the agent was dripping water.
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