306 A. Agassiz — Expedition to the Maldives. 



of the Miladummadulu bank, especially near the northern 

 extremity of the plateau where many of the atolls, such as 

 Hamimadu, Filadu, Kelai and others are fully as large as some 

 of the best known atolls in the Pacific. 



The formation of fresh water or brackish sinks edged with 

 mangroves* in some of the atolls can be traced to the same 

 process which has formed the enclosed lagoons such as we have 

 just described. They occur on Kendikolu, Ekasdu, Nallandu, 

 Maddelu and Filadu. The sinks differ from the lagoons only 

 in being shallow, having been cut off by spits and bars extend- 

 ing across portions of the adjacent reef flats covered only by 

 water of a couple of feet or more in depth, while the enclosed 

 lagoons were cut off from lagoons of atolls of considerable 

 depth, six to seven fathoms or more. What has been written 

 above seems to me to point to the uselessness of our present 

 definition of atolls. There is every possible gradation between 

 a curved open crescent-shaped bank of greater or less size and 

 an absolutely closed ring of land surrounding a lagoon without 

 direct communication with the sea. The evidence of a great 

 number of atolls scattered on an extensive bank or plateau like 

 that of Tiladummati and Miladummadulu shows that reef 

 corals will grow upon any foundation where they find the 

 proper depth, and that local conditions will determine their 

 existence as fringing reefs, barrier reefs or atolls. In fact, in 

 the Maldives, reefs that once formed an atoll may in time, 

 when the atoll is changed into an island, become fringing reefs, 

 a transformation which is quite common both on the outer 

 lines of islands or on islands in the interior of the smaller 

 plateau. The so-called composite atolls of the Maldives are 

 merely elevations upon the greater Maldive plateau which 

 have given to the reef-building corals a base at the proper 

 depth from which they have risen to the surface. In such 

 smaller plateaus as North Male, Ari and others, there is found 

 on the secondary plateaus in their turn a number of bases on 

 which the atolls have grown. In the central and most of 

 the northern plateaus the conditions of exposure to oceanic 

 currents is such that an immense body of water is constantly 

 flowing across the plateau during both the northeast and the 

 southwest monsoons. Where the plateaus are smaller or not 

 as open to the flow of currents as in such atolls as Addu, 

 KardiAa, Goidu, Gaffaru, Wattara, Makunudu, and others we 

 have only a single atoll developed. And again in such plateaus 

 as these upon which Kolumadulu, Haddummati and Suvadiva 

 have developed, the conditions are oceanic (if we might call 

 them so), more similar to those we have in the widely separated 

 atolls of the Ellis, Gilbert or Marshall islands. At the same 



* Messrs. Gardiner and Willis have already called attention to the scarcity 

 of mangroves on the Maldives. 



