CORAL-REEFS, ATOLLS, AND ISLANDS. 



'35 



nothing of it remains above water, except, perhaps, what was 



once the top of some hill or mountain, and that after a 



while this, too, disappears; then nothing will remain to mark 



the spot where the island was but the reef which once 



encircled it. For the coral 



will have continued to grow 



upwards as its foundations 



sank. Had there been a 



sudden plunge into the 



depths below, it would have 



been killed; but this slow, 



gentle subsidence, so far 



from being injurious, merely 



gives it the necessary room 



to grow, and the deeper the 



foundation sinks, the thicker 



the reef will be. 



The whole of the Pacific 

 Ocean is scattered with 

 atolls (Fig. 27), or rings 



of coral, some of them many leagues across ; and all, larger 

 or smaller, are so many memorial stones, marking the site 

 of a buried island. The reef is below water, of course, but 

 its position is clearly marked by the line of snow-white 

 breakers, which are incessantly dashing themselves against 

 it. By their means heaps of coral sand are gradually formed 

 and piled upon the reef, forming little islets which are 

 heaped higher and higher, converted into dry land, and 

 covered with vegetation. It is not often, however, that the 

 whole atoll is raised above the water ; generally speaking 



Fig. 27. GROUND PLAN OF KEEL- 

 ING ATOLL. (After Darwin.} 



