Sect. I. DISTRIBUTION OF CORAL-REEFS. 83 



round the islands in the Grulf of Gruinea. This perhaps 

 may be attributed to the sediment brought down by 

 the many rivers debouching on that coast, and to the 

 extensive mud-banks which line great part of it. But 

 the islands of St. Helena, Ascension, the Cape Verdes, 

 St. Paul's, and Fernando Noronha, are, also, entirely 

 destitute of reefs, although they lie far out at sea, are 

 composed of the same ancient volcanic rocks, and have 

 the same general form with those islands in the Pacific, 

 the shores of which are surrounded by gigantic walls of 

 coral-rock. With the exception of Bermuda, there is 

 not a single coral-reef in the central expanse of the 

 Atlantic ocean. It will, perhaps, be suggested that 

 the quantity of carbonate of lime in different parts of 

 the sea may regulate the presence of reefs. But this 

 cannot be the case, for at Ascension, the waves, charged 

 to excess, precipitate a thick layer of calcareous matter 

 on the tidal rocks; and at St. Jago in the Cape Verdes, 

 carbonate of lime not only is abundant on the shores, 

 but it forms the chief part of some upraised post- 

 tertiary strata. The apparently capricious distribution, 

 therefore, of coral-reefs, cannot be fully explained by 

 any of the above obvious causes ; but, as the study of 

 the terrestrial and better known half of the world, 

 must convince every one that no station capable of 

 supporting life is lost, — nay more, that there is a 

 struggle for each station between different organisms, 

 — we may conclude that in those parts of the inter- 

 tropical sea in which there are no coral-reefs, there 

 are other organic beings, supplying the place of the 



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