6 PROFESSOR T. MCKENNY HUGHES, M.A 



floor of the cave, or is even undercut a little, because the 

 talus has always been removed from the base, so that the 

 fragments broke away all over the face of the cliff from top 

 to bottom, and the base sometimes was even undermined by 

 the waves. 



In the case of an inland cliff, on the contrary, the fallen 

 rock is not removed, so that only the upper part of the 

 cliff above the sloping mass of talus is exposed to the action 

 of the weather. The exposed part is reduced in height as the 

 talus grows, so that the cliff keeps on receding above only, 

 as the talus keeps covering up more and more of the lower 

 part. 



The form that a chalk cliff would eventually have behind 

 the talus has been calculated by the Rev. 0. Fisher.* 



Of course, the sea-cliff, when removed inland by elevation, 

 gets, after a time, eaten back by sub -aerial weathering, and 

 covered over by talus like any ordinary escarpment. 



Gaping fissures of such a character that they could in any 

 case be looked upon as caves are very rare, but the fault- 

 breccia that commonly fills such cracks is easily removed, and 

 the various denuding agencies are apt to follow fissures, and 

 thus caves be formed along them. The unequal flow of lava 

 curling and coiling over the half-cooled mass of earlier flows 

 sometimes leaves openings like caves. 



It is said that some of the caves in volcanic districts are 

 opened out by the various acidic vapours which act on the 

 micaceous and other schistose rocks which have been already 

 fissured by the earthquakes so frequent in those countries 

 as, for instance, in the case of some of the caves of Corinth 

 and the Cyclades.f 



These are, however, few and unimportant, seldom 

 occurring where a cave would be much frequented by man or 

 the lower animals. 



The commonest caves and those which generally 

 have proved of greatest interest, are the old subterranean 

 watercourses so frequent in limestone rocks. The way in 

 which these caves are formed is well known, but many of the 

 phenomena connected with them appear to be less clearly 

 understood, and so we hear of various startling theories pro- 

 pounded which, on inquiry, turn out to be based on a wrong 

 interpretation of the mode of formation of the deposits found 



* Geol Mag., vol. iii. 1866, p. 354. See also Davison, Geol Mag. vol. 

 1886, p. 65. 

 t Virlet, Bull. Soc Geol. de France, i. ii. p. 329. 



