PLEISTOCENE CAVE-DEPOSITS. 85 



caves are buried under considerable accumulations of gravel, 

 sand, and earth or clay. The presence of these deposits shows 

 that sometimes the caves after they had been resorted to by 

 animals again became the channels of engulphed streams, either 

 intermittently or for long continuous periods; while, in other 

 cases, they were ever and anon liable to be inundated by floods 

 carrying into them quantities of mud and silt. Moreover, it 

 can be shown that sheets of stalagmite have occasionally been 

 broken up and removed from certain caves, in whole or in 

 part, so that we cannot always be sure that this may not have 

 happened in the case of many other caves. 



Throughout all the cave-deposits occur, more or less fre- 

 quently, large and small angular fragments of limestone that 

 have evidently fallen from the sides and roof. Sometimes these 

 are scattered pretty equally through the floor-accumulations, at 

 other times they are perhaps more numerous at some levels than 

 at others. They seem also to be present most abundantly in the 

 chambers or galleries that open directly to the day, or which can 

 be shown to have formerly had some such direct connection 

 with tbe external atmosphere. It is also to be noted that the 

 uppermost layer in which any traces of Pleistocene mammals 

 and Palaeolithic man are met with, is not unfrequently sprinkled 

 with numerous fallen masses, and sometimes with a more or less 

 thick breccia of large and small fragments of limestone, by which 

 the mouth or entrance to the cave is occasionally blocked up. 



The fragments may have been detached from the roof in 

 various ways. It cannot be doubted that, as Mr. Pengelly has 

 pointed out, 1 the gradual widening of the joints in limestone by 

 the corrosive action of percolating water must occasionally loosen 

 large blocks, and allow these to fall away ; and as percolation is 

 always going on, such accidents as the sudden dislodgment of 

 fragments may take place at any moment, in any part of a 

 cave, and under any conditions of climate. Again, it is not 

 improbable, as some have suggested, that the tremor of the 

 ground during an earthquake might shake down many half- 



1 Trans. Devon. Assoc, vol. vii. 1875, p. 315. 



