386 THE BOTTOM OF THE SEA. 



insects have finished their work, man appears, and 

 builds his hut on the now fertile soil. 



The coral insects, unable to live in fresh-water, 

 are interrupted in their work wherever a river pours its 

 tribute into the sea. The reefs are also subject to very 

 sudden breaks at a short distance from the sea, if the 

 bottom be a very steep incline ; such are the coast 

 reefs, or broken reefs, so called on account of their 

 situation and of tlieir frequent breaks. 



Sometimes a channel of considerable width, and of 

 more or less depth, separates the reef from the coast. 

 It is then called "a barrier reef." Some of these are 

 of very great extent. One, on the coast of New 

 Caledonia, is 100 leagues long ; another follows the 

 eastern coast of Australia for a distance of 400 

 leagues, almost without interruption. The channel 

 which separates this reef from the mainland is from 

 60 to 100 feet in depth, and its width yaries between 

 15 and 50 leagues. 



Coral reefs are circular when the coast opposite to 

 which they are built is that of a small island. If, 

 instead of an island, there is a shallow, such as would 

 be produced by the summit of a submarine mountain, 

 the reef becomes converted into an island, in the form 

 of a ring, in the midst of which is a lagoon, generally 

 communicating with the ocean, altiiough occasionally 

 it is quite enclosed. Sometimes the lagoon fills up, and 



