LAGOON FORMATION. • 321 



presence or absence of this gas. The greater average and the lesser range of the temperature 

 of the sea-water as compared with more northern and southern atolls (comp. Chagos 

 Archipelago) would probably be an additional factor favourable to coral growth in this region. 

 Lastly, destructive organisms of all kinds are relatively scarce in this lagoon. The larger 

 boring animals are rarely found in the coral skeletons. Indeed in our record of dredgings 

 I do not see a single form recorded, although in contrast to most atolls much rubble was 

 obtained. This is further confirmed by a few corals, which we brought home from the area, 

 in some pieces even the minuter boring sponges being absent. Holothurians and Sipunculids, 

 the two most important sand-triturating forms in such an area, were singularly scarce both 

 on the reefs and in the lagoon, so that there would be in the sea-water a lesser quantity 

 of finely-suspended matter to be removed by the outgoing cuiTents. 



On the whole Addu undoubtedly owes the shoaling of its lagoon rather to want of 

 solution than to the luxuriant growth of organisms, but, the greater the latter, the smaller 

 are the solvent surfaces on which the sea-water can act. That Addu really had a considerably 

 greater depth when Moresby's survey was made, and possibly still greater at a previous period, 

 admits of no doubt. The reason of the relatively sudden filling in of its lagoon can only, 

 I think, be ascribed to increase of land and decrease of depth in the passages, both tending 

 to hinder the free circulation of the sea-water. Against this must be set the possible decrease 

 of the area, from which sediment is supplied for filling in the lagoon. It appears to me 

 doubtful, though, whether this would actually be the case or whether the effective source 

 of sediment would not rather be enlarged with increase of land. The factor, however, is of 

 small importance compared to the prevention of circulation in the water and hence of solution. 



Finally, I may venture to remark that the physical conditions at this latitude are such 

 that it is not unreasonable to suppose that the encircling reef of Addu atoll might have 

 grown up from a greater depth than would be possible in the more northern parts of the 

 Maldive group. The area of the shoal being small, the rim too might have been perfect 

 from the first, and the lagoon would consequently * have had a greater depth. In any case 

 there is nothing in the existing conditions against the supposition that the lagoon may have 

 been at one time greatly deepened by solution, while at present it is decidedly being filled 

 in by coral growth and deposition of sediment. 



Everywhere else in the Maldives such change as there has been in the lagoons has 

 been one rather of loss than gain. In the first place the soundings show in the larger 

 atolls an increase rather than a decrease. In dredging, a hard smooth bottom, free fi'om 

 rubble, was constantly found. The lagoon shoals generally fall almost perpendicularly to 20 

 and sometimes even to 30 fathoms, depths at which they could scarcely have originated in 

 a lagoon. Great falls of rock from their sides are not infrequently found, while the amount 

 of rubble around them is relatively small and forms but a very narrow circle. Boring 

 organisms— so scarce in Addu atoll — are present in extraordinary abundance in other lagoons, 

 and of great importance in destroying corals. The presence of much dead and decaying coral 

 would perhaps provide an abundant supply of carbonic acid gas for solution on the lagoon 

 floor, a position where much could not be absorbed by the corals. 



The circulation of water in all the other atolls of the group is much greater than at 

 Addu owing to their relatively much more extended, peripheral reefs, to which must be added 

 the lesser percentage of their surface covered with land. The passages into their lagoons 

 are generally both far more numerous and deeper, consequently producing a gi-eater amount 



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