92 CONDITIONS FAVOURABLE TO Ch. IV. 



larger, with many wide passages communicating with 

 the open sea. On the other hand, of those atolls in 

 which all, or nearly all the lagoon-reefs have reached 

 the surface, some are small, others large, some shallow, 

 others deep, some well enclosed, and others open. 



Captain Moresby informs me that he has seen a 

 French chart of Diego Garcia made eighty years before 

 his survey, and apparently very accurate ; and from it he 

 infers, that during this interval there has not been the 

 smallest change in the depth on any of the knolls within 

 the lagoon. It is, also, known that during the last fifty- 

 one years, the eastern channel into the lagoon has 

 neither become narrower, nor decreased in depth ; and 

 as there are numerous small knolls of living coral within 

 it, some change might have been anticipated. Moreover, 

 as the whole reef round the lagoon of this atoll has been 

 converted into land — an unparalleled case, I believe, in 

 an atoll of such large size, — and as the strip of land is for 

 considerable spaces more than half a mile wide — also a 

 very unusual circumstance, — we have the best possible 

 evidence that Diego Grarcia has remained at its present 

 level for a very long period. With this fact, and with 

 the knowledge that no sensible change has taken place 

 during eighty years in the coral knolls, and considering 

 that every single reef has reached the surface in other 

 atolls, which do not present the smallest appearance of 

 being older than Diego Grarcia and Peros Banhos, and 

 which are placed under the same external conditions 

 with them, one is led to conclude that these submerged 

 reefs, although covered with luxuriant coral, have no 



