THE CRUST OF THE EARTH. 



Fig. 77. 



the superior ones undisturbed and, consequently, overhanging. 



When this action is continued to a certain point, the cliffs thus 



overhanging fall by their weight (fig. 78). 



It sometimes happens that the accumulation of the debris of 

 such cliffs which takes place below 

 ^hem, operates as a barrier to the 

 waves, and so, for a time, protects 

 them from further degradation (fig. 79). 

 In some cases the natural form of the 

 rocks exposed to the action of the 

 waves enables them to resist these 

 effects, and such forms are accord- 

 ingly often imitated in the construction 

 of harbours and breakwaters (fig. 80). 

 166. Cascades have often the same effect in the degradation 



of the cliffs over which they fall, as have the action of waves 



directed against their base. When such cliffs are formed of 



Fig. 78. 





alternate calcareous and argillaceous matter, the former, being 

 more susceptible of disintegration than the latter, absorbs and is 



Fig. 79. 



Fig. 80. 



worn away by the water either in its fall, or by its recoil from the 

 foot of the cascade ; the cliff, therefore, over which the torrent falls 

 soon overhangs (fig. 81) and at length is broken off by its weight. 

 120 



