72 PREHISTORIC EUROPE. 



portion of our great limestone -caves are simply the deserted 

 channels of subterranean streams and rivers. Many such water- 

 courses are well known at the present day, and the direction of 

 some of them can be traced by the swallow-holes, chasms, and 

 "sinks," which indicate where the roofs of the cavities have 

 given way, or have been pierced by acidulated water. In certain 

 regions almost all the drainage is thus conducted underground — 

 rivers after flowing for a considerable distance at the surface 

 suddenly disappear below the ground, and follow a hidden course 

 for it may be many miles before they reappear. Sometimes, 

 indeed, they never come to the surface again, but enter the sea 

 by subterranean channels. Should anything occur to interrupt 

 such a system of underground drainage, and the streams and 

 rivers be compelled into new channels, the old subterranean 

 courses will then become more or less dry galleries, which may 

 be accessible by one or even by several openings. And although 

 in time these entrances may become blocked by the fall of 

 debris, yet atmospheric erosion — rain, frost, and running- water — 

 will by and by open up new ones, either by the gradual disinte-- 

 gration and removal of rock, or by inducing more or less sudden 

 falls and landslips. 



It would be interesting to ascertain, if that were possible, 

 the age or date of origin of our great limestone-caves. But that, 

 it is to be feared, we shall hardly be able to accomplish. We 

 may indeed infer with much probability that some caves are 

 older than others, but no geologist can say with certainty at 

 what particular time the larger caverns in this and other countries 

 were first open to the light of day, and fitted to become the 

 resort of wild beasts and men. Ever since the limestones have 

 been permeated by water trickling down from the surface, the 

 excavation of caves has been going on, and there is no reason, 

 therefore, why some of the underground galleries in the more 

 ancient limestones (such as those of Devonian and Carboniferous 

 age) may not date back to a period anterior to the deposition 

 of the younger Tertiary formations. Nevertheless, it is remark- 

 able that none of our great caverns has yet yielded deposits of 



