ACTION OF THE WAVES AND OF TIDES. 133 



peared, and others have been cut off and separated from the main 

 land. These effects are more rapid in places where a deep sea 

 swallows up the detached blocks, or in those where the force of the 

 waves is sufficiently powerful to break up the debris, and wear 

 them one against the other and successively remove them, so that 

 the foot of the escarpment always remains bare. 



23. When masses of debris falling from precipices are not im- 

 mediately removed, a natural rampart is formed against the action 

 of the waves, which break before 



reaching the foot of the escarp- 

 ment (Jig. 213); then it is only 

 in a long time that the debris are 

 worn, rounded, and carried away 

 little by little, depending on the 

 solidity of the rocks of which 

 they are formed. These natural 

 ramparts are imitated as much as 

 possible by piling rocks before 

 the ta'lus we wish to preserve on Fig. 213. Accumulation of debrit 

 sea-coasts or river banks. opposing the action of waves. 



24. To the action of waves must be attributed certain excava- 

 tions frequently found, on a level with the sea, in calcareous preci- 

 pices, as well, perhaps, as the arches of greater or less height 

 which traverse certain promontories. Nevertheless, this action 

 does not immediately produce great results, except on matters 

 easily disaggregated, such as chalk, clay, and arena'ceous sub- 

 stances, and it is infinitely slow on more compact and harder sub- 

 stances : in fact, there are points where no effect whatever has 

 been produced within historic times. The erosive power of water 

 does not explain all these facts, nor even the impetuous force of 

 waves ; the soils on which this power is exerted are cracked in all 

 directions, either by previous action, or at the moment of earth- 

 quakes, accompanied by violent agitations of the sea, and it is 

 then they yield to the combined forces to which they are exposed. 

 By this means we can account for isolated rocks, for islands in the 

 vicinity of continents, for those great gaps through which the sea 

 finds passage, for those groups of split rocks which form shoals in 

 the midst of the sea, and for all those severings so common and 

 varied on the coasts of France and England, in numerous islands 

 that extend towards the North Sea, and in a great many localities 

 (figs. 214, 215). 



25. Deposits of detritus formed by wafers. Although waters 

 continually degrade certain parts of 'the globe, they create in a 

 measure new deposits proportioned to those they remove. Tor 



23. What circumstance protects coasts from the action of waves ? 



24. What effects are attributable to the action of waves? 



25. How are deposits formed from water ? 



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