rOEMATIOi^ OF BANKS AND DUNES. 167 



into a lagune. This outline of the shores is represented by the ac- 

 companying illustration, where the heights are strongly exaggerated'. 



Fig. 68.— Section of sea-shore. 



Notwithstanding the looseness of the materials which compose them, 

 the banks are more solid than the promontories of rocks against which 

 they rest, and when the cliffs have been levelled by the waves, the 

 banks of sand again extend from one ledge to the other. They can be 

 displaced by the influence of the currents and the winds, but they do 

 not the less continue apparently immovable and more durable than the 

 mountains. They do not, however, present a continuous development. 

 When the inland bay is fed by one or several rivers, the mass of water 

 which is discharged in this closed basin must necessarily break a 

 passage to the sea, and pierce this ridge at the spot where it offers 

 least resistance ; that is to sa}^, most often at one of its extremities. 

 A remarkable example of this phenomenon is to be seen in Corsica at 

 the mouth of the Liamone. In countries where the year comprises a 

 dry period and a rain}^ season, most of the lagunes on the coast are 

 alternately completely separated from the sea, and united with it by 

 temporary embouchures of inconsiderable depth. When the mass of 

 rain waters has flowed away, the breaches in the broken bank are in- 

 stantly restored by the waves. In the same manner on the shores 

 of seas with strong tides, a number of rivers are alternately canals of 

 almost stagnant water, which a bank of sand separates from the ocean, 

 and vast estuaries up w^hich a powerful tide from the open sea flows. 

 Thus, the Bidassoa, separated from the gulf at low water by a most 

 graceful curved sand-bank, is at the hour of high water an arm of 

 the sea, from 2 to 3 miles wide. Almost all the small water-courses 

 which discharge themselves into the Atlantic are alternately rivers 

 and marshes twice a-day. Even the Orne itself, whose large delta 

 spreads like a fan beyond the coast-line, is lost in a shingle-bank at 

 the hour of low w^ater. 



