Sect. II. 
ATOLLS. 
4 3 
covered by the sea only at high-water. When de- 
scribing South Keeling atoll, I endeavoured to show 
how slow the final process of filling up a lagoon must 
be ; nevertheless, as all causes do tend to produce this 
effect, it is very remarkable that not one instance, as I 
believe, is known of a moderately sized lagoon being 
filled up even to the low-water line at spring-tides, 
much less of such a one being converted into land. It 
is, likewise, in some degree remarkable, how few atolls, 
except small ones, are surrounded by a single linear 
strip of land formed by the union of separate islets. 
We cannot suppose that the many atolls in the Pacific 
and Indian oceans all have had a late origin, and yet 
should they remain at their present level, subjected 
only to the action of the sea and to the growing powers 
of the coral, during as many centuries as must have 
elapsed since any of the earlier tertiary epochs, it can- 
not, I think, be doubted that their lagoons and the 
islets on their reef, would present a totally different 
appearance from what they now do. This considera- 
tion leads to the suspicion that some agency (namely, 
subsidence) comes into play at intervals, and renovates 
their original structure. 
