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. In all cases the original
box work has been in thin sheets of calcite, and the heavy varieties are
due to later deposits of calcite and aragonite crystals or, pop corn.

The colors are white, yellow, blue and chocolate brown; the last named
predominating to a great extent in that portion of the cave most easily
traveled by visitors, and forming the ceiling and a part of one wall in
the Post Office, where, as has been said before, it first appears. The
effect is not dreary as might be imagined, and parties are generally
photographed here because one side of the room is white and greatly
assists the flash. This is a smooth, perpendicular wall marking the line
of the fissure and showing the strata of the rock in horizontal position
whitened with a thin coating of carbonate of lime. All visitors are
cordially invited to please themselves in leaving cards, letters or
papers in this chamber, which is reserved for that purpose, and to
refrain from leaving them in other portions of the cave or defacing the
walls with names.