148 



MANUAL OF ZOOLOGY. 



Pacific Ocean. According to Darwin, coral-reefs may be 

 divided into three principal forms viz., Fringing-reefs, Barrier- 

 reefs, and Atolls, distinguished by the following characters : 



1. Fringing-reefs (fig. 54, i). These are reefs, seldom of 

 great size, which may either surround islands, or skirt the 

 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. Barrier-reef s (kg. 54, 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 





Fig. 54. Structure of coral-reefs, i. F ringing-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. 



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 



