CORAL-REEFS. 93 



at Peros Banhos they rise from a depth of about 

 thirty fathoms, and some of them in the Great Chagos 

 Bank from above forty fathoms; they are covered, Capt. 

 Moresby informs me, with living and healthy coral, two and 

 three feet high, consisting of several species. Why then 

 have not these lagoon-reefs reached the surface, like the 

 innumerable ones in the atolls above-named? If we 

 attempt to assign any difference in their external conditions, 

 as the cause of this diversity, we are at once baffled : the 

 lagoon of Diego Garcia is not deep, and is almost wholly 

 surrounded by its reef; Peros Banhos is very deep, much 

 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. 



Capt. 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 con- 

 verted into land — an unparalleled case, I believe, in an atoll 

 of such large size, — and as the strip of land is for consider- 

 able spaces more than half a mile wide — also a very 

 unusual circumstance, — we have the best possible evidence, 

 that Diego Garcia has remained at its present level for 

 a very long period With this fact, and with the knowledge, 



