GLACIAL-CONTROL THEORY OF CORAL REEFS. 215 



fended by a living bulwark. The surviving corals would reoceupy 

 the abraded surface as fast as possible, making new increments, which 

 in their turn would be subject to later successful attacks by the waves. 



This hypothesis is supported by the character and relative uni- 

 formity of the depths on the rim. Depths of from 7 m. to 20 m. are 

 exactly of the order expected under the conditions. Much of the sand 

 and mud formed in the partial destruction of the reef must be thrown 

 into the lagoon, and the broad terrace, adjoining the inside of the 

 reef-rim and covered with about 30 m. of water, appears to have been 

 thus formed. 



The Pitt, Speakers, and Saya de Malha banks are under nearly the 

 same climatic conditions as the Great Chagos. The China Sea and 

 the open ocean washing the banks north of Fiji (named in the foregoing 

 table), are famous for destructive typhoons. It seems reasonable to 

 explain the submerged rims of all these crowned banks in the same 

 way. (Figs. 34 A and 34B.) 



However, the correctness of the hypothesis now offered is not fully 

 established until the normal, surface reefs of the Chagos archipelago 

 (e. g., Diego Garcia) and of the F'iji group can be explained. The prob- 

 lem does not now seem to be capable of full solution. Factors other 

 than those just enumerated are important and cannot yet be valued 

 properly. Among them are: the prevailing, as contrasted with the oc- 

 casional, weather conditions ; the kinds and numbers of reef corals at 

 work in the respective seas ; the protection exerted by central islands 

 in the case of the Pacific barrier reefs ; the fetch of the hurricane waves ; 

 and the size of the platforms involved. The last-mentioned feature 

 is doubtless quite important, since the density of the muddy clouds 

 stirred up by waves is directly proportioned to the width of the plat- 

 form, as measured along the line in which the waves are running. 



The Maldi^•es are occasionally visited by hurricanes, coming from 

 the Bay of Bengal. However, such storms are already much weakened 

 by their passage across India or Ceylon, and the heavy swell generated 

 in the l)ay itself can have little effect on the Maldive islands. Hence 

 there is good reason for the abundance of flourishing surface reefs in 

 this archipelago, though it lies so near the Chagos group. '*^ 



47 C. Darwin (Coral Reefs, London, 3rd ed., p. 50 (1889)) quoted Captain 

 Moresby's observation that the southern atolls of the Maldives are more 

 constantly exposed to a heavy surf than are the northern atolls. The most 

 southerly of all, Addu, though only 14 km. long, has a maximum lagoon depth 

 of about 70 m., and is thus much deeper than the other small Maldive atolls. 

 Darwin remarks: " I can assign no adequate cause for this difference of depth " 

 (page 47 of his book). Is it in part due to a more frequent killing of the Addu 

 €orals, in post-Glacial time, by the stirring of lag-oon sediments? 



