layer. Then the water creeps horizontally through the soft stratum,
doing most of its work by solution, until it finds a crevice in the
floor through which it can excavate farther in the downward direction;
so it goes on in the manner of steps until it burrows channels to the
open stream. In time the vertical fall under the sink hole will cut
through the hard layer, when the water, abandoning the first line of
exit, will develop another at a lower level, and so in time it comes
about that there may be several stories of the cave, the lowest being
the last to be excavated. Of the total work thus done, only a small
part is accomplished by the falling of the water, acting through the
boring action of its tools, the bits of stone before mentioned; the
principal part of the task is done by the solvent action of the
carbonated waters on the limestone. In the system of caverns known as
the Mammoth Cave, in Kentucky, the writer has estimated that at least
nine tenths of the stone was removed in the state of solution.

When first excavated, the chambers of a limestone cavern have little
beauty to attract the eye. The curves of the walls are sometimes
graceful, but the aspect of the chambers, though in a measure grand,
is never charming. When, however, the waters have ceased to carve the
openings, when they have been drained away by the formation of
channels on a lower level, there commonly sets in a process known as
stalactitization, which transforms the scene into one of singular
beauty. We have already noted the fact that everywhere in ordinary
rocks there are crevices through which water, moving under the
pressure of the fluid which is above, may find its way slowly
downward. In the limestone roofs of caverns, particularly in those of
the upper story, this ooze of water passes through myriads of unseen
fissures at a rate so slow that it often evaporates in the dry air
without dropping to the floor. When it comes out of the rocks the