82 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 
art. 
: The form that a chalk cliff would eventually have behind 
the talus has been calculated by the Rev. O. Fisher.* 
Of course, the sea-cliff, when removed inland by elevation, 
gets, after a time, eaten back by sub-aérial 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.t 
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. 
+ Virlet, Bull. Soc. Géol. de France, t. ii. p. 329, 
