CORAL-REEFS. 81 



banks of sediment, which have accumulated beneath the 

 surface, round either islands or submerged rocks, they are 

 distinguished with difficulty on the one hand from encircling 

 barrier-reefs, and on the other from atolls. In the West 

 Indies there are reefs, which I should probably have 

 arranged under both these classes, had not the existence 

 of large and level banks, lying a little beneath the surface, 

 ready to serve as the basis for the attachment of coral, been 

 occasionally brought into view by the entire or partial 

 absence of reefs on them, — and had not the formation of 

 such banks, through the accumulation of sediment now in 

 progress, been sufficiently evident. Fringing-reefs some- 

 times coat, and thus protect the foundations of islands, 

 which have been worn down by the surf to the level of the 

 sea. According to Ehrenberg, this has been extensively 

 the case with the islands in the Red Sea, which formerly 

 ranged parallel to the shores of the mainland, with deep 

 water within them : hence the reefs now coating their bases 

 are situated relatively to the land like barrier-reefs, although 

 not belonging to that class ; — but there are, as I believe, in 

 the Red Sea some true barrier-reefs. The reefs of this sea 

 and of the West Indies will be described in the Appendix. 

 In some cases, fringing-reefs appear to be considerably 

 modified in outline by the course of the prevailing currents. 

 Dr. J. Allan informs me that on the east coast of Madagascar 

 almost every headland and low point of sand has a coral- 

 reef extending from it in a S.W. and N.E. line, parallel to 

 the currents on that shore. I should think the influence of 

 the currents chiefly consisted in causing an extension, in a 

 certain direction, of a proper foundation for the attachment 

 of the coral. Round many intertropical islands, for instance 

 the Abrolhos on the coast of Brazil surveyed by Capt. 

 Fitzroy, and, as I am informed by Mr. Cuming, round the 



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