XX INTRODUCTION. 



illustrated in the long line of crescent-shaped atolls occurring on the east 

 side of Miladummadulu from Nalandu south as far as Bomasdu. Such a 

 change, from an open crescentic island flanked by a lagoon to a closed land 

 rim surrounding a lagoon, may take place with considerable rapidity. 

 Bodu Mandu is represented on the chart as an open crescent-shaped 

 island; we found it seventy years later a closed land ring completely sur- 

 rounding a small lagoon with a depth of two fathoms. 



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 

 enclosed lagoons. They occur on Kendikolu, Ekasdu, Nalandu, Madidu, 

 and Filadu. The sinks differ from the lagoons only in being shallow, hav- 

 ing been cut off by spits and bars extending 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, crescent-shaped open 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 or faro is changed into an island, become 

 fringing reefs, — a transformation which is quite common both on the outer 

 lines of islands and on islands in the interior of the larger basins. 



The composite atolls of the Maldives have arisen upon minor 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 and 

 faros have grown. In the central and most of the northern plateaus the con- 



1 Messrs. Gardiner and Willis have called attention to the scarcity of mangroves on the Maldives. 



