92 
CONDITIONS FAVOURABLE TO 
Cn. IV. 
a depth of about thirty fathoms, and some of them in 
the Great Chagos Bank from above forty fathoms : they 
are covered, Captain 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 sur- 
rounded 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. 
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 
ally by Captain Moresby, but are taken from the MS. account, before 
alluded to, of the Chagos Group. 
