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, therefore, 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.]