Sect. II. RATE OF GROWTH. 99 



on one of the islands in the same archipelago, coral- 

 sand was passed through for the first 10 or 11 feet, 

 and then solid reef rock. On one of the Maldiva 

 atolls in the Indian Ocean, Captain Moresby bored to 

 a depth of 26 feet, when his augur broke. He gave 

 me the matter brought up, and it was perfectly white 

 like finely triturated coral-rock. 



In my description of Keeliug atoll, I have ad- 

 vanced some facts showing that the reef has probably 

 grown outwards ; and I found, j ust within the outer 

 margin, the great mounds of Porites and of Millepora, 

 with their summits lately killed, and their sides sub- 

 sequently thickened by the growth of the coral : a 

 layer, also, of Nullipora had already coated the dead 

 surface. As the external slope of the reef is the same 

 round the whole of this and many other atolls, the 

 angle of inclination must result from an adaptation 

 between the growing powers of the coral and the 

 force of the breakers, and their action on the loose 

 sediment. The reef, therefore, could not increase out- 

 wards without a nearly equal addition to every part 

 of the slope, so that the original inclination might be 

 preserved, and this would require a large amount of 

 sediment, all derived from the wear of corals and 

 shells, to be added to the lower part. Moreover, at 

 Keeling atoll and probably in many other cases, the 

 different kinds of coral would have to encroach on each 

 other ; thus the Nulliporae cannot increase outwards 

 without encroaching on the Porites and Millepora 



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