98 THE BOTTOM OF THE SEA. 



that the deposit formed on a chalky shore does not 

 contain a trace of calcareous debris. On the Atlantic^ 

 coasts of France the carbonate of lime found in the 

 littoral deposit comes almost entirely from the shells 

 of molluscs of the existing period. It is composed 

 of angular or slightly-rounded fragments, and it 

 resists destruction much better than the most com- 

 pact limestone. On the other hand, we may see a 

 coast without limestone, like that of Brittany, present 

 nevertheless a rich littoral deposit of carbonate of 

 lime, exclusively derived from the debris of shells. 



As we proceed to a distance from the shore, the 

 depth of the water increases, and the marine deposit 

 changes its physical and chemical properties. Thus, 

 the proportion of carbonate of lime grows larger, and 

 its particles at the same time diminish in size. On the 

 whole, these researches demonstrate that the littoral 

 deposit varies in its character with the hydrographic 

 basin to which it appertains, and with the coasts, 

 above and below the water, upon which it is formed ; 

 in the Ocean, on the contrary, it remains pretty con- 

 stantly the same over a vast extent of surface.* 



* Delesse : Becherches sur le depot littoral de la France (Compter 

 Rendus de I'lnstitut de France, Number for January 28, 18G7, 

 pp. 165 et seq.) 



