50 CORAL-REEFS. 



tides, much less of such a one being converted into land. 

 It is, likewise, in some degree remarkable, how few atolls, 

 except small ones, are surrounded by a single linear strip of 

 land, formed by the union of separate islets. We cannot 

 suppose that the many atolls in the Pacific and Indian 

 Oceans all have had a late origin, and yet should they 

 remain at their present level, subjected only to the action of 

 the sea and to the growing powers of the coral, during as 

 many centuries as must have elapsed since any of the 

 earlier tertiary epochs, it cannot, I think, be doubted that 

 their lagoons and the islets on their reef would present a 

 totally different appearance from what they now do. This 

 consideration leads to the suspicion that some renovating 

 agency (namely subsidence) comes into play at intervals, 

 and perpetuates their original structure. 



Section Third. 



Maldiva archipelago. — Ring-formed reefs marginal and central. — Great 

 depth in the lagoons of the S. atolls. — Reefs in the lagoons all rising 

 to the surface. — Position of islets, and breaches in the reefs with 

 respect to the prevalent winds and action of the waves. — Destrttction 

 of islets. — Connection in the position and submarine foundation of 

 distinct atolls. — The apparent dissever?nent of large atolls. — The 

 Great Chagos Bank. — Its submerged condition and extraordinary 

 structure. 



Although occasional references have been made to the 

 Maldiva atolls, and to the banks in the Chagos group, some 

 points of their structure deserve further consideration. My 

 description is derived from an examination of the admirable 

 charts lately published from the survey of Captain Moresby 

 and Lieut. Powell, and more especially from information 

 which Captain Moresby has communicated to me in the 

 kindest manner. 



