Sect. I. THE GROWTH OF CORAL-REEFS. 91 



having examined the charts of each atoll. In the lagoon 

 of Peros Banhos, which is nearly twenty miles across, 

 there is only one single reef which rises to the surface : 

 in Diego Garcia there are seven, but several of these lie 

 close to the margin of the lagoon, and need scarcely 

 have been reckoned : in the great Chagos Bank there is 

 not one. On the other hand, in the lagoons of some of 

 the great southern Maldiva atolls, although thickly 

 studded with reefs, every one without exception rises to 

 the surface ; and on an average there are less than two 

 submerged reefs in each atoll : in the northern atolls, 

 however, the submerged lagoon-reefs are not quite so 

 rare. The submerged reefs in the Chagos atolls gener- 

 ally have from one to seven fathoms water on them, but 

 some have from seven to ten. Most of them are small 

 with very steep sides ; Y 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, 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 



1 Some of these statements were not communicated to me verbally 

 by Captain Moresby, but are taken from the MS. account, before alluded 

 to, of the Chagos Group. 



