274 



THE OCEAN. 



matter, so that living coral is still near the surface; 

 the superior vigour of the species inhabiting the sea- 

 ward edge, however, has caused that edge to be more 



l!iiH■ili| ,, ' 



Section or Coral Island. 



elevated than the interior, as at d, d; so that the 

 appearance is now that of a rocky isle, diminished in 

 extent, surrounded by a reef at some distance, sepa- 

 rated by the intervention of a shallow channel, e, e: 

 this is exactly the appearance of Tahiti and the 

 larger islands generally, as I shall mention more fully 

 when 1 come to the volcanic formation. The subsi- 

 dence still goes on; and, after a while, the water, 

 /3, /3, is level with the summit of the island, which, of 

 course, is now an island no longer; the growth of the 

 coral has kept pace with the depression, and it is 

 still at the surface, as at 5, 5 ; the more slowly grow- 

 ing species of the interior are still overflowed, and, as 

 the island is submerged in the centre, the water, f, f , 

 is no longer an annular channel, but a round lagoon ; 

 and thus we have an atoll, as at first described. The 

 subsequent process of elevating and clothing the new 

 islets is a rapid one. Chamisso observes, " As soon 

 as it has reached such a height that it remains 

 almost dry at low-water at the time of ebb, the 



