678 ON A THEORY OF ATOLL FORMATION. [June 15, 



around an atoll, is presumed also to occur on the reef as a whole, 

 for it is merely a question of substituting colonies for individual 

 zooids to picture the development of the submerged basin -shaped 

 reef. 



The basin-shaped reef continues to grow upwards until tide 

 limit arrests the growth of its margins. At this stage the waves 

 begin to act upon it and hammer fragment against fragment 

 with the production of a quantity of coral debris at the point of 

 maximum intensity of the waves. This debris becomes cemented 

 into solid breccia by the deposition of calcium carbonate around 

 the particles that compose it. This is the beginning of the breccia 

 platform, and its origin may be looked for upon the windward side ; 

 and on that side it will always remain best developed. 



The breccia platform follows the raised rim of the reef in its 

 development, and forms a level, solid, conglomerate cresaent, upon 

 which 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 Avindward side and to be always 

 most perfectly 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 (Hadley 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 

 platform, and so will take place as a part of the circumfei-ence of a 

 circle or a horse-shoe. At the lee side, the waves will not have 

 sufficient force to construct a breccia platform or pile debris 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 con- 

 stituent islands be perfect atollons. In the Trade area, how- 

 ever, the uniformity of the wind will produce a horseshoe-shaped 

 atoll, elongated in the line of the Avind, 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 frag- 

 ments 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 more and more their crescent 



