90 ZOOLOff T 



at the former level of the sea. Where the island has sunk to 

 the level of the water-line II, the reef appears at the sur- 

 face as at b' 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 canal or lagoon, and /' of the 

 fringing reef. After a farther submergence 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, ow- 

 ing to the agency of ocean-currents. 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 sub- 

 marine peaks. The coral reef has now grown to great di- 

 mensions, and covered almost the entire original island, 

 and though the reef-building coral polyps cannot live below 



Fig. 60. Schematic section of ari island with reefs. 



a point fifteen or twenty fathoms below the surface, yet ow- 

 ing to the slow sinking of the island, they build up the- 

 reef as rapidly as the former subsides, and in this way after 

 many centuries a coral reef sometimes two thousand feet 

 thick may be built up in mid-ocean. 



Semper has called attention to the influence of ocean 

 currents, and their varying strength and direction, in shap- 

 ing the forms of coral islands and reefs ; and Moseley holds 

 nearly the same view ; neither of these authors accepts the 

 theory of subsidence.* 



Coral reefs are mainly confined to the Western and Cen- 

 tral Pacific and the Indian Oceans, and to the Caribbean 

 Sea. None occur on the west coast of North America or of 

 Africa, and only limited patches on the eastern coast of 

 South America. There were palaeozoic reefs, such as the 

 fossil coral reef extending across the Ohio River at Louis- 

 ville. 



* See Semper's Animal Life; Agassiz' Three Cruises of the Blake. 



