O14 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 
Section or Corat IsLanp. 
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 I come to the volcanic formation. The subsi- 
dence still goes on; and, after a while, the water, 
B, 8, 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 6,5; the more slowly grow- 
ing species of the interior are still overflowed, and, as 
the island is submerged in the centre, the water, ¢,¢, 
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 
