A. Agassiz — Expedition to the Maldives. 305 



The small atolls which form the outer rim of the composite 

 atolls owe their existence to the same causes, and their develop- 

 ment can be easily traced from a mere ring which has risen to 

 the surface having more or less extensive flats on which islands 

 or islets, or sand bars have been thrown up. The sea-reef flats 

 of these outer lines of atolls are usually wider than those of 

 the lee-face, and naturally so. 



The increase in size of the islands of the outer line of atolls 

 we find goes on much as we have observed it in the Gilbert, 

 Ellis, Marshall and Paumotu Islands. Small islets or islands on 

 the same reef flats are gradually united by the formation of 

 sand spits on the lee-face of the islands, thus forming bays on 

 the sea-face of an atoll ; the spits gradually approach, become 

 connected, and the filling up of the bay from the sea-face 

 unites adjacent islands, their former disconnected state being 

 indicated merely by a difference in the growth of the vegetation, 

 a distinction which gradually disappears with years. The bay 

 may be formed also on the lee-side by the throwing up on the 

 sea-face of a bar on the edge of the reef flat between separate 

 islands, and the bay may then be filled up both from the lee 

 and weather side and thus unite separate islands or sand bars. 



The existence of lagoons completely shut off from the sea 

 in some of the atolls of the northern part of the Maldives is 

 readily explained by their mode of formation ; this can be traced 

 in all its stages from the time the atoll consists of a crescent- 

 shaped island occupying only a portion of the reef flat of the 

 ring, the reef flat of the rest of the ring still enclosing a com- 

 paratively deep lagoon sometimes 6 to 7 fms. The island throws 

 out spits from the horns of the crescent until there is only a nar- 

 row pass left between them, and finally this gap is closed by a sand 

 or shingle beach and we have the ideal atoll, a closed ring of 

 land enclosing a deep lagoon, which exists so rarely but is always 

 the atoll to which one refers when discussing the coral reef 

 question. These phenomena are well illustrated in the long 

 line of crescent-shaped atolls occurring on the east side of 

 Miladummadulu from Kallandu south as far as Bomasdu. Such 

 a change from an open crescentic island flanked by a lagoon to 

 a closed land rim surrounding a land rim may take place with 

 considerable rapidity. Kodularmandu is represented on the 

 chart as an open crescent-shaped island : we found it, 70 years 

 later, a closed land ring completely surrounding a small lagoon 

 with a depth of two fathoms. Other islands on the same reef 

 flats have greatly increased in size by the extension of sand 

 spits over the shallow reef flats connecting them, and it is easy 

 to trace the amount of this growth and its comparative age by 

 the length of these sand spits or by the quality of the vegeta- 

 tion on the ridge connecting them. This is well shown on such 

 islands as Eddufaru, Milandu and many others on the east face 



