g great chasms below. Volcanic agency also deposits huge roofs of
lava over tracts of ice and snow, and the melting of the latter leaves
empty spaces of vast extent. The neighbourhood of Mount Etna, in Sicily,
has various wonderful caverns of this formation. Landslips and
rock-falls on the surface account for many small grottoes, but water is
the main origin of all the most celebrated caverns of the world.
Underground streams and rivers gradually eat their way along the surface
of their rocky flooring, the carbonic acid in the water acting
chemically on the stone in addition to the wearing force of the element.
Once a shallow channel is worn, new forces set to work to deepen it:
sand, pebbles and grit of all kinds, washed down by the current, grind
and wear away the rock. In course of time great depths are hollowed out,
and if it happens that some obstacle turns the course of the water, and
the river finds a new outlet, a long deep gallery is left dry, and here
and there an apparently bottomless pit where the water has acted on
specially soft stone. From above, also, a steady action of moisture has
been eating away the cliff, adding height to the cavern, as well as
coating its roof and sides with a sparkling substance derived from the
union of water and particles of the limestone, in which caves usually
abound.

Nothing can be more beautiful, when illuminated, than a roof of
stalactites, with ascending pillars of stalagmite often meeting and
forming pillars, like those which will be later on described in the
Mammoth Cave and others. The building of these fairy grottoes is really
a simple matter, but one only possible to the Great Architect to whom a
thousand years are as one day; for a very little bit of one of those
stony icicles would take hundreds of years in formation. Water flowing
above a cave is certain to contain carbonic acid, some given to it by
the atmosphere, and some imparted from decaying vegetation. This water
oozes slowly through the rock, and the carbonic acid in passing
dissolves a mite of lime, carrying it through the roof, to which the
lime adheres whilst the water evaporates. Drop follows drop, each tiny
particle sliding down its fellow, until, as weeks and years and
centuries roll by, a lovely long pendant is formed, known as a
stalactite. Sometimes the drops of acidulated watery lime fall through
the roof by an easier passage, and fall right on to the floor of the
cavern, when an upward process ta