are easily recognized from their floors being rarely much out of
the horizontal; their entrances are all in the same plane, or in a
succession of horizontal and parallel planes, if the land has been
elevated at successive times. From their inaccessible position they have
been rarely occupied by man. Among them Fingal's Cave, on the island of
Staffa, off the south-west coast of Scotland, hollowed out of columnar
basalt, is perhaps the most remarkable in Europe. In volcanic regions
also there are caves formed by the passage of lava to the surface of the
ground, or by the expansion of steam and gases in the lava while it was
in a molten state. They have been observed in the regions round Vesuvius
and Etna, in Iceland and Teneriffe. We may take as an example the Grotto
del Cane ("cave of the dog"), near Pozzuoli, a few miles to the
south-west of Naples, remarkable for the flow of carbonic acid from
crevices in the floor, which fills the lower part of the cave and
suffocates any small animal, such as a dog, immersed long enough in it.

The most important class of caves, however, and that which immediately
demands our notice, is that composed of those which have been cut out of
calcareous rocks by the action of carbonic acid in the rain-water,
combined with the mechanical friction of the sand and stones set in
motion by the streams which have, at one time or another, flowed through
them. They occur at various levels, and are to be met with wherever the
strata are sufficiently compact to support a roof. Those of Brixham and
Torquay and of the Eifel are in the Devonian limestone; those of Wales,
Somerset, the Pennine chain, Ireland, the central and northern counties
of Belgium, Saxony, and Westphalia, of Maine and Anjou, of Virginia and
Kentucky, are in that of the Carboniferous age. The cave of Kirkdale in
Yorkshire, and most of those in Franconia and Bavaria, penetrate
Jurassic limestones. The Neocomian and Cretaceous limestones contain
most of the caverns of France, rendered famous by the discovery of the
remains of the cave-men along with the animals which they hunted; as
well as those of the Pyrenees, the Alps, Sicily, Greece, Dalmatia,
Carniola and Palestine. The cave of Lunelviel near Montpellier is the
most important of those which have been hollowed in limestones of the
Tertiary age. They are also met with in rocks composed of gypsum; in
Thuringia, for example, they occur in the saliferous and gypseous strata
of the Zec