PLEISTOCENE CAVE-DEPOSITS. 71 



formation of such hollows, however, it is not necessary that the 

 strata should consist of unequally-yielding materials. Cliffs of 

 homogeneous composition are often undercut by streams, simply 

 by mechanical erosion, but this action of the running- water is 

 frequently much intensified by the influence of frost. Of the 

 mode of formation of sea-caves it is not necessary to speak. 

 They and the more or less shallow rock-shelters and hollows, 

 that occur in the face and at the foot of inland cliffs and steep 

 slopes, may of course be excavated in almost any kind of 

 rock. 



The second class of caves includes all the most extensive 

 underground galleries, many of which ramify in almost every 

 direction, winding tortuously about, and often opening on either 

 side into similar intricate hollows, which in like manner com- 

 municate with lateral extensions of the same character. All 

 these cavities owe their origin to the action of underground 

 water. The chemical composition of mineral springs might 

 have led us to expect that the more soluble strata must fre- 

 quently be honeycombed and excavated to a very considerable 

 extent, for the amount of mineral matter which many of those 

 springs carry to the surface in solution is simply astonishing. 

 We cannot be surprised therefore when we find that here and 

 there the surface of the ground has subsided, the rocks having 

 been undermined by the continuous action of underground water. 

 Subsidences of tins nature are most commonly met with in 

 districts where the prevailing strata are calcareous, but they also 

 occur in regions where rock-salt is plentiful. But since cal- 

 careous strata are more widely diffused, and as a rule occupy 

 more continuous tracts than any other kind of readily -soluble 

 rock, it is in countries where the former abounds that under- 

 ground cavities attain their greatest development. These have 

 been excavated by the chemical action of acidulated water, 

 assisted doubtless in many cases by contemporaneous and sub- 

 sequent mechanical erosion ; that is to say, the cavities have 

 been enlarged by the filing action of the sand and gravel which 

 the underground streams have swept along. For a large pro- 



