ORIGIN" OF CAVERNS. 



155 



nnd that a peculiar stratum harder than the rest, having 

 resisted this abrading force, has formed the kind of valley de- 

 picted in the accompanying figure (94), presenting a central 

 axis whence the strata dip in opposite directions. 



FIG. 94. 



ORIGIN OF CAVERNS. These have been ascribed too 



FIG. 95. 



generally to the agency ^ of water. It is now ascertained 

 that these chasms are owing to the same causes which have 

 produced fissures and valleys, and that they have been occa- 

 sioned by the fractures and dislocations consequent on the 

 upheaval of the strata. At the same time it is equally clear, 



