74 PREHISTORIC EUROPE. 



very strange indeed if denudation should have removed every 

 Pliocene cave, and at the same time opened out a completely 

 new suite of caverns for the use of the Pleistocene fauna. The 

 appearances presented by the oldest accumulations in our great 

 limestone-caves shows that these latter were not only in exist- 

 ence, hut had attained pretty much their present dimensions 

 before they were resorted to by Palaeolithic man and his con- 

 geners; and we can hardly resist the conclusion, therefore, that 

 many of them must have been as accessible in Pliocene times as 

 they subsequently became. And if they were thus accessible 

 at that early period, it is almost certain that they must at one 

 time have contained accumulations of Pliocene age. ISTo trace of 

 these, however, has yet been detected ; but this need not surprise 

 us, because, as we shall presently learn, there is every reason to 

 believe that the caves have frequently been invaded by running- 

 water, and their floor -deposits broken up and swept away. 

 Before the neighbouring valleys had been excavated to their 

 present depth such accidents would be liable to "occur whenever 

 the streams and rivers rose in flood. We know that some of the 

 valleys in question were deepened to the extent of fifty and 

 even of a hundred feet and more during the Pleistocene Period, 

 so that in the preceding Pliocene Age and in early Pleistocene 

 times the caves opening into these valleys would be more 

 exposed to irruptions of water than they were at a later date. 

 When the Pleistocene Period was far advanced many of the 

 caves seem to have remained permanently dry, and the accumu- 

 lations of floor-deposits continued with little or no interruption. 

 Had the rivers continued to flow at the same level all through 

 the Pleistocene Period, it is more than probable that no con- 

 siderable floor-deposits would have escaped destruction. In the 

 sequel we shall find that there is abundant evidence to show 

 that some of the older cave-accumulations have experienced no 

 little denudation. The caves would appear to have been cleared 

 out again and again. And if this has been the actual fate of 

 Pleistocene accumulations, we need not wonder at the apparently 

 entire absence of Pliocene cave-deposits. It is quite possible, 



