90 ZOOLOGY 
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 ; 0’ 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-levet 
III, the canal e” becomes much wider. On one side (ff) 
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 7’”, 7’”, 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 
BR” oe” 
a A 
Zhe! f= 
f. ee 
Fig. 60.—Schematic section of an 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 paleozoic 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. 
