264 CORAL AND ATOLLS 



the barrier advances. The seaward advance of the barrier 

 edge is of necessity very slow, for a vast amount of submarine 

 building must take place before the wave-washed edge can 

 grow outwards. The barrier edge probably grows outwards as 

 the outcome of unusually high seas, and the evidences that are 

 to be seen of sudden increase of island building have probably 

 been preceded by similar increases in barrier building. With 

 the exception of these uncertain and irregular methods of 

 building the sea is, therefore, powerless to further add to the 

 growth of the island's dry land ; it is the wind that completes 

 the moulding of the pile of coral debris. When the building 

 by coral boulders and fragments ceases, the building by coral 

 sand begins ; in all the stages of the after-history of the atoll, 

 sand is by far the most important factor. The sources of this 

 sand are manifold. Most of the sand is made on the barrier, 

 where the grinding of wave-driven fragments is for ever 

 taking place. On the flats, the broken branches of Madrc- 

 pora and fragments of massive colonies are for ever sweeping 

 backwards and forwards in the waves, and being reduced to 

 smaller and smaller particles. Sand is always being pro- 

 duced all round the atoll ring, wherever rough water moves 

 fragment on fragment ; and vast quantities are always in 

 the process of production, becoming increasingly fine as the 

 waves carry the particles sho rewards. Compared with this 

 method of sand production, it is probable that any others are 

 of quite trivial importance, but of such as do make an appre- 

 ciable contribution, I am convinced that in the Cocos-Keeling 

 atoll, at any rate, the fish of the family Scarus are by far the 

 most important. With their hard beaks these fish are always 

 nibbling the masses of dead coral upon which Nulliporcc and 

 other algse have become encrusted ; they actually bite deep 

 grooves into the rock and the linear scores that they leave 

 upon the boulders are easily identified, although Europeans 

 usually fail to associate them with the fish which undoubtedly 

 produce them. The most important sand-producing species 

 is a blue Scarus — native name Kakatua i/bw— which weighs, 

 as a rule, about five pounds ; this blue fish feeds round the 



