CORAL-REEFS. 41 



is much less at the extremities of the more elongated atolls 

 in the Low Archipelago, than at their sides ; in speaking of 

 Ducie's Island he says 1 the buttress, as it may be called, 

 which " has the most powerful enemy (the S.W. swell) to 

 oppose, is carried out much further, and with less abrupt- 

 ness than the other." In some cases, the less inclination of 

 a certain part of the external slope, for instance of the 

 northern extremities of the two Keeling atolls, is caused by 

 a prevailing current which there accumulates a bed of sand. 

 Where the water is perfectly tranquil, as within a lagoon, 

 the reefs generally grow up perpendicularly, and sometimes 

 even overhang their bases ; on the other hand, on the 

 leeward side of Mauritius, where the water is generally 

 tranquil, although not invariably so, the reef is very gently 

 inclined. Hence it appears that the exterior angle varies 

 much ; nevertheless in the close similarity in form between 

 the sections of Keeling atoll and of the atolls in the Low 

 Archipelago, in the general steepness of the reefs of the 

 Maldiva and Chagos atolls, and in the perpendicularity of 

 those rising out of water always tranquil, we may discern 

 the effects of uniform laws ; but from the complex action of 

 the surf and currents, on the growing powers of the coral 

 and on the deposition of sediment, we can by no means 

 follow out all the results. 



Where islets have been formed on the reef, that part 

 which I have sometimes called the 'flat' and which is 

 partly dry at low water, appears similar in every atoll. In 

 the Marshall group in the North Pacific, it may be inferred 

 from Chamisso's description, that the reef, where islets 

 have not been formed on it, slopes gently from the external 

 margin to the shores of the lagoon : Flinders states that 

 the Australian barrier has a similar inclination inwards, 

 1 Beechey's Voyage, quarto ed., p. 44. 



