cohal reefs. 
36 
reefs of the West Indies are, generally speaking, in an area 
of elevation. 
A section of a coral reef is shown by Fig. 34, B: 7i\^ the 
point where the sliore 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 toward the 
lagoon slopes away regularly from d to 7i\ 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, tlie 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 V /', b /. There is now a 
fringing and a barrier reef, with a narrow canal between 
them; V is a section of the barrier reef, of the canal or 
lagoon, and /' 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 
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 
