CORAL REEFS 87 



rock or of volcanic material, the shore of which, in 

 most cases, is boulder-strewn. The reef will thus form 

 a narrow shelf around the land, being then termed a 

 fringing reef (Fig. 24). Sometimes the land behind 

 is separated from the boulder zone by a channel ; this 

 may be only a few feet in depth, when it is termed a 

 boat channel, or it may be some fathoms deep, sufficient 

 for a ship's anchorage, when it is commonly called a 

 lagoon. To a considerable degree the depth of this 

 lagoon depends on the distance of the land from the 

 surface reef outside, but it never exceeds 50 fathoms. 

 Inside of most reefs, within the boulder zone, there is 

 almost invariably (if no island arises within the limits 



-^^^^^^ 



Fig. 24. — Section of Fringing Reef, Rotuma Islands, 

 Pacific Ocean. 



a, Land ; b, b, boat channel or lagoon flat ; c, c, boulder zone ; d, d, reef- 

 flat ; e, crest or edge of reef ; /, /, seaward slope. 



of this zone) a broad flat, with 1 or 2 feet of water 

 at low tide, more or less covered with sand, part bare, 

 part rich in coral growth. This may be termed the 

 lagoon flat, and may be the flat beneath our boat 

 channel, or may be a flat before the water deepens to 

 form the lagoon. Such a reef, with a deep lagoon 

 within it around the land, is termed a barrier reef, 

 but there is no sharp transition between it and the 

 fringing reef. 



Lastly, the reef may exist as an isolated structure by 

 itself, with its reef flat, boulder zone, and lagoon flat. 

 It may be — (1) a small mass completely swept by the 

 breakers ; or (2) a larger reef with a mere pile of boulders 

 in its centre ; or (3) with the smallest puddle of water 



