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 included deposits generally
get more or less completely cemented together by the percolation
through them of water holding carbonate of lime in solution. If
the cave is only partially filled, the dropping of water from
the roof holding lime in solution, and its subsequent evaporation,
would lead to the formation over the deposits below of a layer of
stalagmite, perhaps several inches, or even feet, in thickness.
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