s of none but living Mammals. The two
other sets of gravels are Post-Pliocene, as they contain the
bones of extinct Mammals, mixed with land and fresh-water shells
of existing species. Among the more important extinct Mammals
of the low-level and high-level valley-gravels may be mentioned
the _Elephas antiquus_, the Mammoth (_Elephas primigenius_),
the Woolly Rhinoceros (_R. Tichorhinus_), the Hippopotamus, the
Cave-lion, and the Cave-bear. Along with these are found
unquestionable traces of the existence of Man, in the form of
rude flint implements of undoubted human workmanship.

The so-called "Cave-deposits," again, though exhibiting peculiarities
due to the fact of their occurrence in caverns or fissures in the
rocks, are in many respects essentially similar to the older
valley-gravels. Caves, in the great majority of instances, occur
in limestone. When this is not the case, it will generally be
found that they occur along lines of sea-coast, or along lines
which can be shown to have anciently formed the coast-line. There
are many caves, however, in the making of which it can be shown
that the sea has had no hand; and these are most of the caves
of limestone districts. These owe their origin to the solvent
action upon lime of water holding carbonic acid in solution.
The rain which falls upon a limestone district absorbs a certain
amount of carbonic acid from the air, or from the soil. It then
percolates through the rock, generally along the lines of jointing
so characteristic of limestones, and in its progress it dissolves
and carries off a certain quantity of carbonate of lime. In this
way, the natural joints and fissures in the rock are widened, as
can be seen at the present day in any or all limestone districts.
By a continuance of this action for a sufficient length of time,
caves may ultimately be produced. Nothing, also, is commoner
in a limestone district than for the natural drainage to take
the line of some fissure, dissolving the rock in its course. In
this way we constantly meet in limestone districts with springs
issuing from the limestone rock--sometimes as large rivers--the
waters of which are charged with carbonate of lime, obtained by
the solution of the sides of the fissure through which the waters
have flowed. By these and similar actions, every district in which
limestones are extensively developed will be found to exhibit
a number of natural caves, rents, or fissures. The first element,
theref