CORAL REEFS 215 



description and does not represent any sharp distinctions in 

 Nature. 



But when a reef is situated only a few score of yards 

 from the shore, and separated from it at low tide by sand- 

 banks and boat channels, it is called a " Fringing reef." 

 When the reef is a mile or more from the coast-line and 

 separated from it by a lagoon with a few fathoms of water 

 at low tide, it is called a " Barrier reef." An atoll is a 

 circular, oval, or more irregular shaped island or chain of 

 islands in the open ocean composed of recent coralline lime- 

 stone raised a few feet above the level of the sea at high tide 

 and fringed on the outer side with coral reefs. 



There are many intermediate forms between these three 

 varieties. Thus a fringing reef at one part of a coast-line 

 may be continuous with a barrier reef further along the 

 coast, and it would be difficult to say at exactly what spot 

 the one type merges into the other. 



In the Paciiic Ocean there are many examples of more 

 or less conical islands surrounded by a barrier reef ; there 

 are cases of a very small central island surrounded by an 

 atoll-like barrier reef, and then there are the more typical 

 atolls without a central island. There is evidently in Nature 

 a complete series of these forms, and there is no sharp 

 distinction in type between a small island with a barrier 

 reef and an atoll. There are also many different kinds of 

 atolls. There is the typical ring-shaped island with a 

 central shallow lagoon ; there is the ring-shaped island with 

 one or more breaks in it, through which the tides rush back- 

 wards and forwards from the lagoon to the open ocean. 

 There are the half-ring or quarter-ring atolls with a group 

 of reefs or islets representing the other parts of the atoll 

 awash at high tide. And then there are the huge banks in 

 the Indian Ocean, one hundred miles or more in length, as 

 seen in the Maldive and Laccadive Archipelagoes, which 

 present the appearance of an atoll of atolls, an enormous 

 ring of atolls enclosing an immense lagoon perched on the 

 edge of a submarine bank that rises from the deep water 

 of the ocean. 



Each of these reefs consists of a great variety of living 



