CORAL REEFS. 35 



reefs of the West Indies are, generally speaking, in an area 

 of elevation. 



A section of a coral reef is shown by Pig. 34, B: n\5 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 the open ocean. Between h c and 

 d e lies the higher part of the reef. The shore towai-d the- 

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

 open ocean there is a broad horizontal terrace [a to h c) 

 ■which becomes uncovered at low water. 



Darwin's theory of the formation of barrier reefs is shown 

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

 canic island Coro, which is slowly sinking, at the ancient 

 sea-level lis surrounded by a fringing reef / /, a small 



Fig. 35. — Schematic section of an island with reefs. 



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

 island has sunk tc the level of the water-line II, the reef 

 appears at the surface as at l' f, b f. There is now a 

 fringing and a barrier reef, with a narrow canal between 

 them; V is a section of the barrier reef, e' of the can:il or 

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

 mergence to the sea-level III, the canal e" 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 

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

 reef has now grown to great dimensions, and covered almost 

 the entire original island, and though the reef-building 

 coral polyps cannot live below a point fifteen or twenty 



