266 CORAL AND ATOLLS 



serious one. It is true that they contain large quantities of 

 sand — indeed Dr. Guppy, who attached much importance to 

 their influence, calculated that fifteen of them passed a ton of 

 sand through their bodies every year. It is quite possible 

 that some such figure may hold true, but I do not regard this 

 ton as any addition to the island's sand capital, for the Holo- 

 thurian has merely passed it through its body. It was sand 

 when it went in and it is sand when it comes out, so I do not 

 think that the Holothurian is a serious rival of the Scarus in 

 this respect or a great agent in sand-production. The influence 

 of the various boring molluscs and worms is again still less, 

 and their importance as sand-producers is trivial indeed, and 

 has, as a rule, I think, been very considerably overrated. In 

 the Cocos-Keeling atoll there are only two agents that produce 

 sand from solid coral rocks in any appreciable quantities ; 

 one is the waves of the barrier and the other is the 

 fish of the genus Scarus, and the influence of these latter 

 has hitherto, I believe, been entirely neglected or very much 

 under- estimated. 



The sand, by whatever method it is produced, is the lightest 

 of the particles that the waves carry — it is the finest product 

 of coral trituration — and it will therefore be the last burden to 

 be deposited by the waves. The boulders are left on the 

 barrier edge ; the smaller fragments are deposited on the barrier 

 flats or added to the island pile ; but the sand is carried high 

 up the seaward beaches, or far out into the lagocn. The sand 

 that is thrown upon the island beaches becomes dried in the 

 sun, and scattered over the island land by the winds. The 

 effects of this wind-blown sand have been seen in the ex- 

 amination of the islands, and the characters of wind-made 

 islands have been noted. 



The islands of the windward side of the reef will be the 

 flrst to obtain an abundant supply of sand, and their rough 

 coral boulders will soonest become clothed by a layer of sand. 

 In places, great mounds of sand will become piled up — these 

 are the atoll mountains, and they may rise to a height of 30 

 feet under favourable conditions. 



