ods the dense growth of tropical vegetation and
multitudinous animal life in the Northwest, the waters necessarily
became heavily charged with the naturally resulting carbonic acid gas,
and this, acting on the limestone rocks, would decompose them, leaving a
residual clay and taking the chief portions of the mineral components in
solution, to be afterwards deposited according to circumstances and
conditions; and these are indicated by the various results found in Wind
Cave, Crystal Cave, the Onyx Caves and the Bad Lands. The latter being
previous to that time by no means "bad," but richly luxuriant in
tropical vegetation, which gave shelter from the heat to great numbers
of curious animals.

Some approximate idea of the extreme age of these caves may be gained
from the fact that bones of a three-toed horse have been discovered in a
chamber of Crystal Cave that must be practically unchanged since the
remains were carried in from the outside, as otherwise they would have
been buried beneath the fallen masses of crystal covered rock with which
the entire floor is cumbered. And yet this room is so remote from any
present connection with the outer world that it is impossible for their
introduction to have taken place in recent times.

In the beautiful Catacombs progress is as slow as in a cactus thicket or
a blackberry patch. The crevices lack none of the usual crevice
irregularities; high places must be mounted or descended, chasms crossed
and narrow passages crawled through, while extra caution must be
exercised to avoid striking the head or making a misstep that might
result in a fall. The hands are in constant use and soon become so
sensitive that holding a soft handkerchief gives infinite relief; but
the worst experience is the "crawls" where only the soles of the feet,
being temporarily turned up, seem safe from the savage treatment of the
sharp calcite dog-teeth. The worst crawl encountered was a small one
having a downward slope with a jump-off at the end which necessitated
its being taken feet first. Fortunately it was short. But in no place do
the difficulties outweigh the pleasure of beholding scenes of such
beauty, or suggest regret for the time, torn garments, and personal
exertion required for its enjoyment.

In many portions of the cave the surface layer of crystals has had the
points worn away by the action of water, later than that in which they
were formed; but in the Catacombs and other extensive regions