170 DISTRIBUTION OF CORAL-REEFS. Ch. VI. 



and Couthouy's observations relate chiefly to the 

 Paumotu atolls, and here again some facts indicate 

 recent subsidence rather than elevation : I refer to the 

 manner in which Chain atoll suffered during a storm, 

 and to Sir E. Belcher's statement, 1 that after an interval 

 of fourteen years, a well-known islet had disappeared, 

 and the lagoon at a particular spot had become deeper 

 than it was before. 



There are other causes of change which might, as it 

 appears to me, easily lead to a mistaken belief in the 

 recent elevation of low coral formations. We must re- 

 member that the outer and living margin of the reef 

 grows up to a height determined by the constant break- 

 ing of the waves. Outside this margin there is a sloping 

 surface also covered with living corals, but belonging to 

 species which do not grow to the surface ; and beyond 

 this, there is a much steeper slope consisting of coral- 

 sand. Now after a somewhat rapid subsidence of, for 

 instance, one or two fathoms, we may feel almost sure 

 that the corals on the outer margin would grow up 

 quickly to the surface and form a nearly vertical wall. 

 This would be succeeded outside by a steeply sloping 

 surface of living corals, which would likewise sooner or 

 later grow up in their former level ; but outside this, 

 the much steeper slope, formed by the slow accumula- 

 tion of fine detritus, would not recover for a very long 

 time its former angle relatively to the upper bank of 

 living corals. Now it seems highly probable that a 



1 Voyage Bound the World, vol. i. 1843, p. 382. 



