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MANUAL OF ZOOLOGY. 



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. 



b <* 



Fig. 36. Structure of coral-reefs, i Fringing-reef ; 2 Barrier-reef; 3 Atoll, a Sea 

 level ; & Coral-reef; c Primitive land ; d Portion of sea within the reef, forming a 

 channel or lagoon. 



2. Barrier-reefs (fig. 36, 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 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 

 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. Atolls (fig. 36, 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 



