46 THE CORAL REEFS OF THE MALDIVES. 



described to such a larger reef flat. Its original large lagoon has been 

 filled up and left a diminutive pool ; the incipient sand-bar has become a 

 constantly growing islet and is covered with scant vegetation derived from 

 the adjoining faros. It needs but an additional stage in the filling of the 

 lagoon and increase in the size of the islet to reach the condition of the 

 island we described lying in the southeast pass to the northeast of Male 

 Island. With the growth of the land larger trees will obtain a foothold, 

 and the original lagoon become transformed into a great reef flat similar 

 to that of the former island. This island rose quite abruptly from a depth 

 of twenty-five fathoms much as did the island to the eastward of Male. 

 It is evident from the mass of broken and dead corals towards the base of 

 the outer slope that the rim flat increases in width by the reaching out 

 of the outer talus. 



Corals obtain a foothold on the upper slope of the flat and form extensive 

 patches reaching over its surface from the outer rim. The reef flat gradu- 

 ally becomes covered with fragments and small masses of coral coated with 

 Nullipores which bind the whole together. 



These rings are apparently formed by the upward growth of circular or 

 elliptical patches of corals occupying slight elevations above the general 

 level of the surrounding plateau. These patches may grow up uniformly, 

 forming elliptical banks, or they may grow as annular structures, the corals 

 of the outer face only rising towards the surface, while those in the central 

 part of the patch are killed either from want of food or of clean sea water, 

 or are choked by the sand derived from the dead corals of the outer rim, 

 which is washed into the interior and covers the corals. The outer ring 

 growing faster than the inner dish is filled, a central lagoon is formed, 

 this is filled in time after the ring rim has reached the surface ; the annular 

 structure is then changed into a bank from which all traces of the original 

 lagoon have disappeared and upon the outer rim of which sand-bars or 

 islets or both have formed. There is thus eventually formed either an 

 island surrounded by a more or less extensive reef flat or an island and reef 

 flat with the remnants of a lagoon, or an island or islet on the rim of a 

 ring with a well-marked and deep lagoon, all different stages of growth of 

 an annular coral reef formed upon a base laid within the range of depths 



