Ch. III. FKINGING EEEFS. 79 



Sea some true barrier-reefs. The reefs of this sea and 

 of the West Indies will he 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. Eound many intertropical islands, for 

 instance the Abrolhos on the coast of Brazil sur- 

 veyed by Captain FitzEoy, and, as I am informed by 

 Mr. Cuming, round the Philippines, the bottom of the 

 sea is entirely coated by irregular masses of coral, 

 which although often of large size, do not reach the 

 surface and form proper reefs. This must be owing 

 either to insufficient growth, or to the absence of those 

 kinds of corals which can withstand the breaking of 

 the waves. 



The three classes, atoll- formed, barrier, and fringing 

 reefs, together with the modifications just described of 

 the latter, include all the most remarkable coral-forma- 

 tions anywhere existing. At the commencement of 

 the last chapter in the volume, where I detail the 

 principles on which the map (Plate III.) is coloured, 

 the exceptional cases will be enumerated. 



