When the storms toward the close of the Tertiary period suddenly overwhelmed with floods 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 as well, the finished work of crystallization is preserved in an absolutely perfect condition.  And everywhere the largest crystals are on the under side of a projection or the roof of a cavity.