FOHMATION OF CAVERNS. 91 



be formed anew in accordance with it. In seas of 

 little extent, such as the Mediterranean or Caspian, 

 and at those points of the ocean where the tidal 

 elevation is but small, the cliff will consist of but 

 one simple escarpment. When, however, the differ- 

 ence between the level of high and low water is 

 considerable, as in the English Channel and other 

 narrow passages, there will be found a second cliff 

 corresponding with the level of low-water. 



Many marine caverns have been formed by the 

 erosive action of the sea upon massive beds of 

 bastilt. This rock is the product of ancient volcanic 

 eruptions, the volcanic matter having separated 

 itself into prismatic columns in the process of cool- 

 ing. The sea, by its reiterated assaults through 

 a long lapse of time, causes the lower beds of basalt, 

 which are the most exposed to the lury of the waves, 

 to yield first. Thus are formed caves, or sometimes 

 extended galleries and halls, of which the Cave of 

 Fingal is a beautiful example. 



The erosive action of waves is slower when the 

 rock is in horizontal beds. In such cases the water 

 rolls over the inclined surface until its course is 

 arrested, and in its return it breaks the force of the 

 next approaching wave. 



A striking effect of the erosive action of the sea is 

 shown in the phenomenon, so frequently met with, of 



