388 THE BOTTOM OF THE SEA. 



barrier reefs, of which atolls are evidently only a 

 particular instance? 



We have already explained that the bottom of 

 the sea is undergoing a constant tliough very slow 

 variation. It rises in some points to sink at others ; 

 and this movement, which requires centuries to make 

 itself evident, is the most energetic agent of the 

 modifications occurring on the surface of our planet. 

 Darwin has very ingeniously applied the knowledge 

 of this general fact to the explanation of atolls. We 

 give his theory in a few words, as follows : 



The coral insects do not live beyond a depth of 

 forty yards. They are therefore confined to the 

 shallows, or to the neighbourhood of coasts. On the 

 other hand, the close proximity of land troubles them 

 for various reasons, among which may be mentioned 

 the outflow of the soft waters of rivers, and the agi- 

 tation which causes mud, &c. to be held in sus- 

 pension. They therefore remain at a certain distance 

 from the shores. Sooner or later the waves break 

 up a poi'tion of the coral barriers, and wash the frag- 

 ments over the reef into the clear channel which it 

 thus tends to fill. When the reef reeiches the surface 

 of the water the existence of its artificers becomes 

 impossible. But if the soil be slowly sinking, there 

 must always remain sufficient water above the colony 

 to permit of its continued growth. 



