270 CORAL AND ATOLLS 



after-Hstory : — it may become joined across the reef flat to 

 its neighbour or neighbours. We have seen that the reason 

 why islands do not normally unite round the edge of the 

 reef is that, as the channel between them narrows, the current 

 of water becomes swifter, so that the deposition of sediment 

 in the channel is impossible. If, for any reason, the current 

 between two islands should slacken down, then the deposi- 

 tion of sediment will begin, and island may stretch across 

 to island; this is, of course, what actually happens. Many 

 things may lead to a slackening of the current in the in- 

 lets ; a storm of unusual violence may cast boulders into its 

 channel, and be powerless to move them farther. Other 

 fragments collect against them, and the channel may become 

 blocked up, the wind-blown sand will then complete the work, 

 and join the islands across the gap. 



The closure may be more gradual, and in this gradual 

 blocking the beds of calcareous algse no doubt play some part. 

 As a rule the inlets to the lagoon are not favourable sites for 

 coral growth, and some stunted colonies of Montipora alone 

 live in them ; but here the only considerable beds of the 

 LitJiothamnionece are found. Within the meshes of the growth 

 of these algse much sand becomes held, and, in many spots, it 

 would seem that they are steadily filling up the channels, and 

 will in the course of a great many years succeed in blocking 

 the waterway. 



Besides the sand that is deposited as continuations to the 

 extremities of the islands, a vast amount is carried on into the 

 lagoon. A part of this goes, as we have seen, to the making of 

 broad beaches to the lagoon shores of the islands, and a 

 general filling in of the lagoon. The sediment borne in the 

 middle of the stream is carried on the farthest, and, with the 

 slackening of the current, it is dropped as banks far out in the 

 lagoon. This point has received some previous notice, and 

 it is mentioned again as it is one of some importance ; the 

 proper understanding of it makes clear some of the changes in 

 the after-history of atolls. As a general principle it may be 

 laid down that the currents in the windward inlets set con- 



