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MANUAL OF ZOOLOGY. 



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

 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 some- 

 times called an " encircling barrier-reef," and it constitutes 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 



Fig. 38.— Structure of coral-reefs. 1 P'ringing-recf ; 2 Barricr-rcef; 3 Atoll, a Sea 

 level; 4 Coral-reef; c Primitive land; (if Portion of sea within the reef, formine a 

 channel or lagoon. 



thousand miles, its average distance from the shore being be- 

 tween 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. A/o/ls (fig. 38, 3).— These are 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 

 from the prevailing winds. In their structure they are iden- 

 tical with "encircling barrier-reefs," and differ from these only 



