DEVELOPMENT OF THE ATOLL 257 



the island starts to form by some boulder being tossed by 

 the waves upon the platform of the barrier. With the advent 

 of any permanent obstruction to the waves a new set of con- 

 ditions is brought into play. 



During my stay in the Cocos-Keeling atoll, I became 

 impressed by the very marked influence of any hindrance to 

 the waves and current, and the changes brought about in this 

 way were fully studied. In the same atoll Dr. Guppy had 

 previously called attention to the facts, and his account was 

 the first that established the real cause of the development of 

 island form. In 1907 Mr. C. Hedley further followed out 

 these processes during his examination of the Great Barrier 

 Keef of Australia, and the whole sequence of events, from the 

 rising of some permanent hindrance to the wash of the 

 currents to the development of an island, or even of an atoll, 

 may be taken as fully established. The steps in the process 

 are very simple, and they may be watched in any stream of 

 running water; for the development of a sand-bank in a 

 river follows very much the same rules as those that apply to 

 the formation of atoll islands. The boulders which the high 

 seas have tossed on the barrier flat stand in the line of the 

 waves that sweep across the flat ; the current strikes them 

 and rushes along their face, and dividing, sweeps on in fan- 

 shaped currents from their extremities. Behind the obstruc- 

 tion an area of calm is created, and here small particles come 

 to rest so that, after a time, a little sand-bank is raised. The 

 currents that become slackened by meeting the obstacle drop 

 their burden of sediment in lines that stream from its ex- 

 tremities, and they tend to raise a V-shaped bank, with the 

 apex of the V at the point of obstruction. As the original 

 obstruction grows larger, by the addition of new fragments 

 washed against it, the area of slackened current increases, 

 more and more sediment becomes deposited, and a bank of 

 coral debris and coral sand is piled upon the barrier. This is the 

 nucleus of the island, and such nuclei may be seen at many 

 points of a barrier ring. 



When the nucleus has attained a somewhat greater 



