CORAL REEFS. 





reefs of the West [ndies are, generally speaking, in an 

 of elevation. 



A section of a coral reel is Bhown by I' 2 34, //: n ie the 

 point where the shore slopes rapidly down within the la- 

 goon (which lies to the right), and m is where the reef sud- 

 denly descends toward tlie ope ■••.■in. Between b c and 



d e lies the higher pari of the reef. The shore toward the 

 on slopes away regularly from d to n; while toward the 

 open ocean there is a broad horizontal terrace (atobc) 

 which becomes uncovered at low water. 



I >ai'w in's theory of the formation of barrier reefs is shown 

 1)\ the diagram (Fig. 35). The island, for example, the vol- 

 canic island C'oro, which is slowly sinking, at the ancient 

 sea-level I is surrounded by a fringing reef f f, a small 



ii 'ii of an island « uli i ■ 



-terrace at the former level of the sea. Where the 

 island has sunk t< the level of the water-line II. the reef 

 appears at the Burface as at b' /''. h f. There i- now- a 



fringing aid a harrier reef, with a narrow canal between 

 them; V is a section of the harrier reef, e' of the canal «>r 

 lagoon, and f of the fringing reef. After a farther sub- 

 mergence to the sea-level III. the canal /' becomes much 

 wider. On one side (//) the reef is present, on the other 

 side it has disappeared, owing to the agency of ocean-cur- 

 rents. Finally, at the water-level IV. there are two small 

 islands surrounded by a wide lagoon, with two reef-islets 

 /'", /'", resting upon two submarine peaks. The coral 

 reef has now grown to great dimensions, and covered aln 

 the entire original island, and though the reef-building 

 COral polyps cannot live below a point fifteen or twenty 



