116 



MANUAL OP ZOOLOGY. 



shores of continents. These shore-reefs have no channel of 

 any great depth intervening between them and the land, and 

 the soundings on their seaward margin indicate that they 

 repose upon a gently sloping surface. 



2. 



a 



3. ^^vivv=:^jjj 



Pig. 31. Structure of coral reefs. 1. Fringing-reef. 2. Barrier-reef. 3. Atoll. 

 a. Sea level ; b. Coral-reef ; c. Primitive land ; d. Portion of sea within 

 the reef, forming a channel or lagoon. 



2. Barrier-reefs (fig. 31, 2). These, like the preceding, may 

 either encircle islands, or may skirt continents. They are 

 distinguished from fringing- reefs by the fact that they occur 

 usually at a much greater distance from land, that there inter- 

 venes a channel of deep water between them and the shore, and 

 that soundings taken close to their seaward margin indicate 

 enormous depths. If the barrier-reef surround an island, it 

 is sometimes called an ' encircling barrier- reef,' and it con- 

 stitutes with its island what is called a * lagoon-island.' 



As an example of this class of reefs may be taken the great 

 barrier-reef on the N.E. coast of Australia, the structure 'of 

 which is on a perfectly colossal scale. This reef runs, with a 

 few breaches in its continuity, for a distance of more than a 

 thousand miles, its average distance from the shore being 

 between twenty and thirty miles, and the depth of the inner 

 channel being from ten to sixty fathoms, whilst the sea out- 

 side is 'profoundly deep' (in some places over 1,800 feet). 



3. Atolls (fig. 31, 3). These are nearly circular reefs of 

 coral, enclosing a central expanse of water or lagoon. They 



