174 CORALS AND CORAL ISLANDS. 



abrupt than that to windward, but facts thus far obtained are 

 not sufficiently definite or extensive to settle this question. It 

 is probably true, yet the difference, if any, must be small. 



III. STEUCTURE OF CORAL ISLANDS. - 



The descriptions of reefs and their islets already given apply 

 with equal force to coral islands. By transferring here the 

 statements respecting the former, we should have a nearly 

 complete account of the latter. The same causes, with scarcely 

 an exception, are at work : — the growing of coral zoophytes, and 

 the action of the waves, of oceanic currents, and of the winds. 

 This resemblance will be rendered more apparent by a review 

 of their characters. The description will be found to be a sim- 

 ple recapitulation of a former paragraph. 



The reef of the coral atoll, as it lies at the surface still 

 uncovered with vegetation, .is a platform of coral rock, usually 

 two to four hundred yards wide, and situated so low as to be 

 swept by the waves at high tide. The outer edge, directly 

 exposed to the surf, is generally broken into points and 

 jagged indentations, along which the waters of the resurging 

 wave drive with' great force. Though in the midst of the 

 breakers, the edge stands a few inches, and sometimes a foot, 

 above other parts of the platform ; the incrusting JSfullipores 

 cover it with varied tints, and afford protection from the 

 abrading action of the waves. There are usually three to five 

 fathoms water near the margin ; and below, over the bottom, 

 which gradually deepens outward, beds of corals are growing 

 profusely among extensive patches of coral sand and frag- 

 ments. Generally the barren areas much exceed those flour- 

 ishing with zoophytes, and not unfrequently the clusters are 

 scattered like tufts of vegetation in a sandy plain. The grow- 

 ing corals extend up the sloping edge of the reef, nearly to 



