Sect. II. MALDIVA ATOLLS. 43 



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 can- 

 not, 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 considera- 

 tion leads to the suspicion that some agency (namely, 

 subsidence) comes into play at intervals, and renovates 

 their original structure. 



SECTION THIRD. 



Maldiva Archipelago — Ring-formed reefs, marginal and central — Great 

 depth in the lagoons of the southern atolls — Beefs in the lagoons all 

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

 respect to the prevalen t winds and action of the waves — Destruction 

 of islets — Relation in position between distinct atolls — The apparent 

 disseverment of large atolls — The Great Chagos Bank — Bs 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 consi- 

 deration. My description is derived from an exami- 

 nation of the admirable charts lately published from 

 the survey of Captain Moresby and Lieut. Powell, 

 and more especially from information which Captain 



