1 < L DISTRIBUTION OF CORAL-REEFS. Ch. YI< 
of the corals. Here, so far from there having been 
any recent elevation of the land, we have reason to be- 
lieve that there has been subsidence. Messrs. Dana 
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 SirE. 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 to 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 
1 Voyage Round the World, vol. i. 1843, p. 382. 
