e, but chemistry
teaches us that water charged with carbonic acid gas will readily
dissolve it. Rain-water falling from the clouds is sure to come in
contact with masses of decaying vegetable matter, which we know is
constantly giving off quantities of this gas. Laden with this the water
sinks into the ground, and, if it comes in contact with limestone,
readily washes some of it away in solution. But beds of limestone rock
are noted for containing great fissures through which subterranean
waters penetrate far into the ground. We can readily see how this
percolating water would dissolve and wear away the surface of the
rocks along such a fissure, and in process of time we would have the
phenomenon of a stream of water flowing under ground.

Owing to a great many causes--such, for instance, as the meeting of
another fissure--we would expect that portions of this underground way
would become enlarged to spacious halls. In some such a way as this it
is now understood that all caves have originated.

Owing to many natural causes the river may, after a while, cease to
flow, leaving enlarged portions of its channel behind as a succession
of chambers in a cave. But water would still come trickling in from
the tops and sides, and be continuously dripping to the floor, where
it speedily evaporates. When such is the case it leaves behind it the
limestone it held in solution. So, in process of time, if the deposition
is undisturbed, there will be formed over the floor of the cave a more
or less continuous layer of limestone matter known as stalagmite. The
same formations on the top and sides of the cave are called stalactites.
In places where the drip is continuous the stalactite gradually assumes
the shape of an immense icicle; while the stalagmite on the floor of
the cave, underneath the drip, rises in a columnar mass to meet the
descending stalactite. A union of these is not uncommon, and, we have
pillars and columns presenting the strange, fantastic appearance on
which tourists delight to dwell in their notes of travel.

While these accumulations are in all cases very slow, still we can
not measure the time since it commenced by the rate of present growth,
because this rate varies greatly at different times and places even
in the same cave. And we must also remark that this complete series
of changes only occur in a few localities, the majority of caves being
insignificant in size.<2>

From what has been said as to the form