403 



development, and forms a level, solid, conglomerate crescent, upon v^hich 

 the waves break at low tide. Upon this platform some waves of unusual 

 violence will hurl fragments broken from the reef margin, and these 

 masses will be left stranded upon the platform when the force of the 

 waves can trundle them no further. 



This is the beginning of the island, and this process also may be 

 expected to originate at the windward side and to be always most per- 

 fectly developed there. Any fragment thrown upon the breccia platform 

 is potent to bring about an important change , for it initiates a process 

 that may be seen anywhere when an obstacle is placed in the line of 

 a current of water that carries any sediment in its stream. The current 

 impinges on the impediment and its burden of sediment is deposited in 

 stream lines from its extremities (Hedley and Dr. Guppy). In this 

 way the form of the island tends to become that of a crescent. 



The piling up of fragments will follow the line of the breccia plat- 

 form , and so will take place as a part of the circumference of a circle 

 or a horse-shçe. At the lee side, the waves will not have sufficient force 

 to construct a breccia jolatform or pile débris upon it, so the lagoon 

 entrance is situated upon this side. When the wind blows in opposite 

 directions for two definite seasons, as in the Monsoon area, the action 

 may be equalised all round the reef edge, and so the atoll be a completed 

 ring and each of its constituent islands be perfect atollons. In the Trade 

 area, however, the uniformity of the wind will produce a horseshoe- 

 shaped atoll, elongated in the line of the wind, with crescentic islands 

 on its windward side. When the atoll structure is once developed, the 

 enclosed lagoon tends to become the resting-place of a vast amount of 

 sediment, formed by the disintegration of coral fragments by the force of 

 the waves. The method of the deposition of this sediment is important. 



As waves rush over the breccia platform in the intervals between 

 adjacent islands, the current becomes slowed at the sides of the inlet, 

 and sand is deposited in stream lines from the extremities of the islands, 

 helping to increase their crescent form. In the middle of the interval 

 between two islands the inrushing current sweeps on farthest, and 

 its burden of sand is dropped in the lagoon opposite the gap in the 

 island ring. 



This process accounts for the existence of those atolls that have 

 the most land upon their leeward side, and an entrance guarded by a 

 breccia platform upon their windward side. The sand swept in at 

 their windward side is deposited upon the lee side of the lagoon (if it 

 be a small one) and comes to rest in the original lagoon entrance. The 

 entrance becomes blocked up, and a wide belt of land is formed upon 

 the lee side of the atoll; but no barrier reef exists upon the lee side. 



26* 



