96 ON CORAL REEFS AND ISLANDS. 



these views. These points or capes correspond to points in the 

 original land, and often to the line of the prominent ridge ; and 

 it is well known that snch ridge-lines often extend a long dis- 

 tance with slight inclination compared with the slopes or de- 

 clivities bounding the ridge on either side. 



Coral islands or reefs often lie in chains like the peaks of a 

 single mountain range: — for example, the sickle-shape line of 

 islets north of Nanuku. Taritari and Makin, (Tarawan group, 

 see map at the end,) lie together as if belonging to parts of one 

 island. Menchicoff atoll, in the Caroline Archipelago, consists 

 of three long loops or lagoon islands, united by their extremi- 

 ties, and further subsidence might reduce it to three islands.* 



The sizes of atolls offer no objection to these views, as they 

 do not exceed those of many barrier reefs. Some of the larger 

 Maldives, according to the crater theory, would require a crater 

 seventy miles in diameter, with a rim made up of subordinate 

 craters. No hypothesis of such extravagance is necessary. The 

 facts all fall in with known principles, and are illustrated by known 

 and established truths, without hypotheses of any kind. 



It is of some interest to follow still further the subsidence of a 

 coral island, the earlier steps in which are illustrated in the pre- 

 ceding figures. One obvious result of its continuation is a grad- 

 ual contraction of the lagoon and diminution of the size of the 

 atoll, owing to the fact already noted, that the detritus is mostly 

 thrown inward by the sea. The lagoon will consequently be- 

 come smaller and shallower, and the outline of the island in 

 general more nearly circular. Finally, the reefs of the different 

 sides may so far approximate by this process, that the lagoon is 

 gradually obliterated, and the large atoll is thus reduced to a 

 small level islet, with only traces of a former depression about 

 the centre. Thus subsidence is connected with detritus accu- 

 mulations in filling up the lagoon ; and as filled lagoons are found 

 only in the smallest islands, such as Swains and Jarvis, the two 

 agencies have beyond doubt been generally united. 



This subsidence, if more rapid than the increase of the coral 

 reef, becomes fatal to the atoll, by gradually sinking it beneath 

 the sea. Of this character evidently is the Chagos Bank.f The 

 southern Maldives have deeper lagoons than the northern, fifty 



* See Darwin on the probable clisseverment of the Maldives, op. cit., p. 37, in 

 •which he points out indications of a breaking up of a large atoll into several smaller. 

 A land with many summits or ranges of heights may at first have its single enclos- 

 ing reef; but as it subsides, this reef contracting upon itself may encircle separately 

 the several ranges of which the island consisted, and thus several atoll reefs may 

 result in place of the large one ; and further, each peak may finally become the 

 basis of a separate lagoon island, under a certain rate of subsidence or variations in 

 it, provided the outer reef is so broken as to admit the influence of waves and 

 winds. The Maldives are a good example of this result. Some of the large atolls 

 are properly atoll archipelagos. 



f For a detailed account of this and other submerged reefs, see Darwin, p. 106. 



