THE QUATERNARY PERIOD. 



343 



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 

 a river- valley, as in fig. 256, the river must have excavated 



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 communi- 

 cate 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. 



its channel to such a depth that it can no longer wash out the 

 contents of the cave even in high floods. 



If the cave be entirely filled, the included deposits generally 

 get more or less completely cemented together by the percola- 

 tion through them of water holding carbonate of lime in solu- 

 tion. 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. In this way cave-deposits, with their con- 

 tained remains, may be hermetically sealed up and preserved 

 without injury for an altogether indefinite period of time. 



In all caves in limestone in which deposits containing bones 

 are found, we have then evidence of three principal sets of 

 changes, (i.) A period during which the cave was slowly 

 hollowed out by the percolation of acidulated water ; (2.) A 

 period in which the cave became the channel of an engulfed 

 river, or otherwise came to form part of the general drainage- 

 system of the district; (3.) A period in which the cave was 

 inhabited by various animals. 



As a typical example of a cave with fossiliferous Post- 

 Pliocene deposits, we may take Kent's Cavern, near Torquay, 

 in which a systematic and careful examination has revealed the 

 following sequence of accumulations in descending order : 



(a) Large blocks of limestone, which lie on the floor of the 

 cave, having fallen from the roof, and which are sometimes 

 cemented together by stalagmite. 



(b) A layer of black mould, from three to twelve inches 

 thick, with human bones, fragments of pottery, stone and 



