72 CORAL ISLANDS. 
In pure water of a certain temperature the deposition of 
lime by corals is very rapid. Coral Islands, or a/o/ds, are, as 
a rule, nearly circular or horse-shoe shaped, the inner Jagoon 
being shallow and communicating with the open sea by a 
channel on the leéward side. There is usually deep water 
off the island. Coral Reefs are small and skirt the shore of 
an island, frequently as a long ridge parallel to the shore and 
some distance from it when they are called Fringing Reefs, 
or if they be large and a long way out from the shore 
they are called Barrier Reefs. The water outside of a 
barrier reef is often of great depth. 
Fig. 22.—D1aGram To ILLUSTRATE DaRwin’s THEORY OF 
CoraL REEFs. 
Third level of sea 
with Atoll 
Second level of sea 
with barrier reef. 
First level of 
_ sea with 
fringing reef, 
The slow subsidence of the land causes successive changes of sea-level. 
The difficulty in accounting for the origin of coral 
islands and coral reefs lies in the fact that the commonest 
and best reef-builders do not find suitable conditions of 
temperature below about 25 fathoms, whereas great depths 
are found immediately outside atolls and barrier reefs. This 
is overcome by various suppositions. That connected with 
the name of Darwin assumes extensive subsidence of the 
land, gradually converting a fringing reef round a peak 
into an atoll, the process being so slow that the coral is 
always built up to the surface, whereas that which passes 
below the 25-fathom line ceases to grow, owing to death 
of the animals. Other theories hold that a deeply-submerged 
