THE QUATERNARY PERIOD. 355 



present day in any or all limestone districts. By a continu- 

 ance 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, ob- 

 tained 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 lime 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 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 neighboring river. Sometimes 

 the cave has been the bed of an ancient stream, and the de- 

 posits 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 excava- 

 tion of its bed to the present depth (fig. 256). In this last 

 case, the cave-deposits obviously bear exactly the same rela- 

 tion 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 dis- 

 trict 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 configura- 

 tion 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 



