WEARING AWAY OF THE COASTS. K9 



called a geogenic basin, to distinguish it from a 

 geographic basin. 



Thus, the basins of the Rhone, the Ebro, the 

 Arno, the Tiber, and several other rivers, all com- 

 bine to form one vast geogenic basin. The Tyrriie- 

 nian Sea is one of the divisions of this great basin, 

 the Gulf of Lyons is another, that of Genoa a third. 



2. Action of Waves upon the Coasts — Destruction of Rock-bound 

 Shores by the Sea — Pierced Rocks — Silting-up of Shallow 

 Waters by Mariue Alluvium. 



In the case of a steep shore the erosive action of 

 the sea is considerable. It beats against the rocky 

 barrier with all its force. The base of the clifif, in- 

 cessantly attacked by the waves, is of course eaten 

 away with a rapidity proportioned to the ease with 

 which the matter of the rock can be disintegrated. 

 The upper part of the cliff, though not subjected 

 to the direct action of the waves, falls forward, and 

 occasionally forms deep rocky caverns such as we 

 see at Bonifacio. At length the superincumbent and 

 tottering mass falls into the sea. If the depth be 

 great, or the current strong, the accumulated debris 

 is swept away, and the action of the .waves against 

 the broken cliff is continued with undiminished in- 

 tensity. In this manner entire promontories have 

 been destroyed ; even within the historic period, the 



