ter have been originally
subterranean watercourses, which have been unroofed by the degradation
of the rock. In all limestone districts ravines are to be found
continued in the same direction as the caves, and the process of
atmospheric erosion may be seen in the fallen blocks of stone which
generally are to be met with at the mouths of the caverns. In
illustration of this the valley and caves of Weathercote, in Yorkshire,
may be quoted, or the source of the Axe at Wookey; and the ravine formed
in this way has very frequently been widened out into a valley by the
action of subaerial waste, or by the grinding of glaciers through it
during the glacial stage of the Pleistocene period.

For further details as to the physical history of caverns we must
refer the reader to the works quoted at the end of this article, by
E.A. Martel, the intrepid explorer of most of the large European
caves, including those of Great Britain and Ireland. The history of
the _Glacieres_ or Ice-caves will be found in Browne's _Ice Caves in
France and Switzerland_.

_Classification._--The caves which have offered shelter to the
_mammalia_ are classified according to their contents, and are of
various ages, ranging from the Pliocene to the present day. (1) Those
containing the Pliocene _mammalia_ belong to that age. (2) Those with
the remains of the mammoth, woolly rhinoceros and other extinct species,
or with paleolithic man (see ARCHAEOLOGY), are termed Pleistocene. These
are sometimes called Quaternary, under the mistaken idea that they
belong to an age succeeding the Tertiary period. (3) Those which contain
the remains of the domestic animals in association with the remains of
man either in the Neolithic, Bronze or Iron stages of civilization are
termed Prehistoric. (4) The fourth group consists of those which can be
brought into relation with the historic period, and are therefore termed
Historic.

_The Pliocene Caves._--It is a singular fact, only to be explained by
the vast denudation of the earth's surface since the Pliocene Age, that
only one cave referable to that age has as yet been discovered, that at
Doveholes near Buxton, Derbyshire, described by Boyd Dawkins in 1903
(_Quart. Journ. Geol. Soc._). The cave consists of a large horizontal
chamber and a small passage, connected with a swallow-hole close by, and
exposed in the working face of a quarry in 1901, at a depth of about 40
ft. from the surface. The locality is in th