THE AFTER-HISTORY OF THE ATOLL 269 



the lagoon. It is easily seen how the forces that shape the 

 whole atoll into a horse-shoe, or an incomplete circle, are acting 

 on the individual islands, and that the incurving of the ends 

 of the islands may easily bring about the partial enclosing of 

 a portion of the lagoon between the horns. Horse-shoe-shaped 

 islands are thus produced, and by the deposition of sand-bars 

 they come to enclose a private lagoon of their own. This 

 process is well seen in the Cocos atoll, where all stages of the 

 evolution are at hand. 



Each island of the reef therefore tends to become atoll- 

 shaped, and to enclose its own little lagoon. In the Maldives 

 this process is seen in great completeness, and it has advanced 

 so far that, in the case of many reefs, a series of small atolls 

 stand ringed about a central lagoon. When an atoll lies in 

 the region of the Trade winds, it would seem as though the 

 islands to the lee side of the reef could never become 

 .atoUons, for although their extremities are incurved by the 

 water running through the inlets, the wash of the lagoon waves 

 would always be likely to lead to the unmaking of banks, 

 when the lagoon was sufficiently large to afford any sort of 

 sea. The Maldives are in the monsoon zone, and therefore get 

 a seasonal change of wind : but I notice, from the charts, that 

 the western atollons are most complete ; and I would suspect 

 the west monsoon to have the greatest influence on their 

 building. The final stage of the process that leads to in- 

 dividual islands enclosing their own lagoons is the filling in of 

 the area that they have enclosed. Many agencies come into 

 play in this, and the most important is again the wind-blown 

 sand. The agent of second importance is the vegetation ; and 

 crabs of the genus Gelasmus, and Cardiosoma, also play their 

 part, very much after the fashion of earth-worms. A fine 

 white mud is produced in these little lagoons by the decom- 

 position of the coral sand; bushes of PempMs acidida, coco- 

 nut palms, and coarse grass take root in the mud; sand 

 is driven about their roots, and the whole area is slowly 

 reclaimed. 



One other fate may befall the island in the course of its 



