ore, in the production of cave-deposits, is the existence
of a period in which limestone rocks were largely dissolved, and
caves were formed in consequence of the then existing drainage
taking the line of some fissure.

Secondly, there must have been a period in which various deposits
were accumulated in the caves thus formed. These cavern-deposits
are of very various nature, consisting of mud, loam, gravel,
or breccias of different kinds. In all cases, these materials
have been introduced into the cave at some period subsequent to,
or contemporaneous with, the formation of the cave. Sometimes
the cave communicates with the surface by a fissure through which
sand, gravel, &c., may be washed by rains or by floods from some
neighbouring river. Sometimes the cave has been the bed of an
ancient stream, and the deposits have been formed as are fluviatile
deposits at the surface. Or, again, the river has formerly flowed
at a greater elevation than it does at present, and the cave
has been filled with fluviatile deposits by the river at a time
prior to the excavation of its bed to the present depth (fig.
256). In this last case, the cave-deposits obviously bear exactly
the same relation in point of antiquity to recent deposits, as
do the low-level and high-level valley-gravels to recent
river-gravels. In any case, it is necessary for the physical
geography of the district to change to some extent, in order
that the cave-deposits should be preserved. If the materials
have been introduced by a fissure, the cave will probably become
ultimately filled to the roof, and the aperture of admission
thus blocked up. If a river has flowed through the cave, the
surface configuration of the district must be altered so far
as to divert the river into a new channel. And if the cave is
placed in the side of a river-valley, as in fig. 256, the river
must have excavated its channel to such a depth that it can no
longer wash out the contents of the cave even in high floods.

[Illustration: Fig 256.--Diagrammatic section across a river-valley
and cave. _a a_, Recent valley-gravels near the channel (b) of
the existing river; c, Cavern, partly filled with cave-earth;
_d d_, High-level gravels, filling fissures in the limestone,
which perhaps communicate in some instances with the cave, and
form a channel by which materials of various kinds were introduced
into it; _e e_, Inclined beds of limestone.]

If the cave be entirely filled, the include