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MANUAL OF ZOOLOGY. 



2. Barrier reefs (fig. 91, 2). These, like the preceding, may 

 either encircle islands, or may skirt continents. They are dis- 

 tinguished from fringing-reefs by the fact that they occur usu- 



Fig. 91. Structure of coral-reefs, i. 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. 



ally at a much greater distance from land, that there intervenes 

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

 soundings taken close to their seaward margin indicate enor- 

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

 sometimes called an "encircling barrier-reef," and it consti- 

 tutes 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 outside 

 is " profoundly deep" (in some places over 1800 feet). 



3. Atolls (fig. 91, 3). These are oval or nearly circular reefs 

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

 seldom form complete rings, the reef being usually breached 

 by one or more openings, which are always situated on the lee- 

 ward side, or on that side which is most completely sheltered 



