212 J. S. Gardiner — Origin of Coral Reefs. 



growth. The effect will be absolutely opposite, if the outward 

 growth of the atoll causes a considerable broadening of the 

 rim. Much land on the encircling reef, by preventing the 

 rush of tidal water into the lagoon, by protecting the lagoon 

 from wind, etc., may prevent a free circulation of the water 

 for solution and erosion, and so produce a stillness and calm- 

 ness in its waters, eminently favorable for a vigorous growth 

 of corals which may to a greater or less extent fill it up, a 

 condition perhaps found in the lagoon of Addu atoll. 



Lastly, land in the Maldives appears to have been formed 

 by three means, (1) a small elevation, (2) the washing back and 

 piling up of loose coral masses on a reef flat, and (3) the wash- 

 ing and blowing up of sand on a flat from the lagoon. Most 

 of the islands on the rim reefs, which show any definite signs 

 of their origin appear to have been formed in their earliest stage 

 by the first means, but they have frequently been very greatly 

 added to and increased in size by the last. At the present 

 time the islands show in one place increase and in another loss, 

 the latter on the whole predominating. 



The various changes and modes of growth, as sketched above, 

 may be seen in the different banks and atolls of the Maldive 

 Group. The open banks show numerous small flat reefs, aris- 

 ing from 25-30 fathoms, and even complete little atolls stud- 

 ding their surfaces. A flat reef may become completely 

 covered with land, or a small depression may appear in its 

 center due to the impossibility of organisms growing in that 

 position, to solution and the outwash of sand and mud. This 

 may deepen, and, the whole meantime spreading, our bank 

 may turn into a small atoll (atollon or faro), while other neigh- 

 boring faro may perhaps have grown up as such. Our atollons 

 will still go on broadening, and this will especially be the case 

 with those towards the edges of the bank, since they will be 

 bathed by the fresliest water. As growth proceeds, the reefs 

 of these atollons will meet and fuse with one another, forming 

 a double rim to our bank, which, by this means, will be turned 

 into an atoll {vide Suvadiva Atoll). Finally the inner parts of 

 the same little atolls will be removed by solution and the 

 action of the currents, so that their little lagoons will be 

 thrown into the big central lagoon, and thus will be brought 

 about the formation of our typical atoll. The distribution, the 

 slopes and topography of these small faro and banks of the 

 Maldives, and the distribution of sedentarj^ and other life in 

 the group are explicable on such a view alone, though there is 

 not wanting evidence also from the comparison of the present 

 condition of the banks with that shown in Moresby's charts of 

 1836. The latter evidence is in any case quite subsidiary and 

 much of it of doubtful value on account of the small scale of 



