hstein, and in the gypseous Tertiary rocks of the
neighbourhood of Paris, as, for example, at Montmorency.

Caves formed by the action of carbonic acid and the action of water are
distinguished from others by the following characters. They open on the
abrupt sides of valleys and ravines at various levels, and are arranged
round the main axes of erosion, just as the branches are arranged round
the trunk of a tree. In a great many cases the relation of the valley to
the ravine, and of the ravine to the cave, is so intimate that it is
impossible to deny that all three have been produced by the same causes.
The caves themselves ramify in the same irregular fashion as the
valleys, and are to be viewed merely as the capillaries in the general
valley system through which the rain passes to join the main channels.
Sometimes, as in the famous caves of Adelsberg, Kentucky, Wookey Hole in
Somersetshire, the Peak in Derbyshire, and in many in the Jura, they are
still the passages of subterranean streams; but very frequently the
drainage has found an outlet at a lower level, and the ancient
watercourses have been deserted. These in every case present
unmistakable proof that they have been traversed by water in the sand,
gravel and clay which they contain, as well as in the worn surfaces of
the sides and bottom. In all districts where there are caves there are
funnel-shaped depressions of various sizes called pot-holes or
swallow-holes, or betoires, "chaldrons du diable," "marmites des
geants," or "katavothra," in which the rain is collected before it
disappears into the subterranean passages. They are to be seen in all
stages, some being mere hollows which only contain water after excessive
rain, while others are profound vertical shafts into which the water is
continually falling. Gaping Ghyl, 330 ft., and Helln Pot in Yorkshire,
300 ft. deep, are examples of the latter class. The _cirques_ described
by M. Desnoyers belong to the same class as the swallow-holes.

The history of swallow-holes, caves, ravines and valleys in calcareous
strata may be summed up as follows:--The calcareous rocks are invariably
traversed by joints or lines of shrinkage, which are lines of weakness
by which the direction of the drainage is determined; and they are
composed to a large extent of carbonate of lime, which is readily
exchanged into soluble bicarbonate by the addition of carbonic acid. The
rain in its passage through the air takes up carbonic ac