CORAL-REEFS. 17 



Here, as in Whitsunday Island, the whole of that part of 

 the reef which is visible is converted into land. This is a 



circumstance of rare occurrence; more usually a snow- 

 white line of great breakers, with here and there an islet 

 crowned by cocoa-nut trees, separates the smooth waters of 

 the lagoon-like channel from the waves of the open sea. The 

 barrier-reefs of Australia and of New Caledonia, owing to 

 their enormous dimensions, have excited much attention : 

 in structure and form they resemble those encircling many 

 of the smaller islands in the Pacific Ocean. 



With respect to fringing, or shore-reefs, there is little in 

 their structure which needs explanation ; and their name 

 expresses their comparatively small extension. They differ 

 from barrier-reefs in not lying so far from the shore, and in 

 not having within a broad channel of deep water. Reefs 

 also occur around submerged banks of sediment and of 

 worn-down rock ; and others are scattered quite irregularly 

 where the sea is very shallow ; these in most respects are 

 allied to those of the fringing class, but they are of com- 

 paratively little interest. 



I have given a separate chapter to each of the above 

 classes, and have described some one reef or island, on 

 which I possessed most information, as typical ; and have 

 afterwards compared it with others of a like kind. Although 

 this classification is useful from being obvious, and from 

 including most of the coral-reefs existing in the open sea, it 

 admits of a more fundamental division into barrier and 

 atoll-formed reefs on the one hand, where there is a great 



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