DEVELOPMENT OF ATOLLS. 
2S5 
wliile the reef is still several fathoms below the surface, 
the corals in the central part are placed at a disadvantage, 
which becomes greater as they are left behind in the up- 
ward race by their neighbours. In a small reef, the peri- 
phery for the supply of food to the interior is relatively 
large ; thus the lagoons in small atolls are also small and 
are soon filled up, while long and narrow banks have no 
lagoons. As the reef becomes larger the conditions 
become more favourable to the formation of lagoons, for 
(as is shown by experiment) the lagoon of such an atoll 
is less rich in pelagic life than the exterior water. Thus 
growth is checked ; many species of coral die, and their 
calcareous ‘ skeletons ’ are exposed to the solvent action 
of sea-water. When the water outside becomes too deep 
for reef-building corals to live, the debris from the exist- 
ing reef, aided by the accumulation of organisms, forms a 
talus at the foot of its submarine cliffs, and thus the reef 
spreads slowly outward, ‘ like a fairy ring,’ on foundations 
to which its own materials have contributed. Coral-reefs 
which have been elevated for some distance above sea- 
level are frequently found to rest upon a deposit thus con- 
stituted. 1 The lagoon channels have in many cases been 
subsequently formed by the solvent action of sea-water, and 
the islets in the lagoon channel are parts of the original 
reef still left standing. Where the reefs rise quite up to 
the surface and are nearly continuous, there is little coral 
growth in the lagoon or its channels ; where the outer reefs 
are much broken up the growth is relatively abundant. 
* At the Admiralty Islands, on the lagoon side of the 
islets of the barrier-reefs, the trees were found overhanging 
the water, and in some cases the soil was washed away from 
their roots. It is a common observation in atolls that the 
islets on the reefs are situated close to the lagoon shore. 
1 The case of Tahiti is here described ; see p. 314, where it is dis- 
cussed by Prof. Dana. 
20 
