CORAL-REEFS. 99 



atolls, Capt. Moresby bored to a depth of twenty-six feet, 

 when his auger also broke : he has had the kindness to give 

 me the matter brought up ; it is perfectly white, and like 

 finely triturated coral-rock. 



In my description of Keeling atoll, I have given some 

 facts, which show that the reef probably has grown out- 

 wards j and I have found, just within the outer margin, 

 the great mounds of Porites and of Millepora, with their 

 summits lately killed, and their sides subsequently 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 atoll, and round 

 many other atolls, the angle of inclination must result from 

 an adaption 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 outwards, 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 corals would have to encroach 

 on each other ; thus the Nulliporse cannot increase outwards 

 without encroaching on the Porites and Millepora com- 

 planata, as is now taking place ; nor these latter without 

 encroaching on the strongly branched Madrepora, the Mille- 

 pora alcicornis, and some Astrseas ; nor these again without 

 a foundation being formed for them within the requisite 

 depth, by the accumulation of sediment. How slow, then, 

 must be the ordinary lateral or outward growth of such reefs. 

 But off Christmas atoll, where the sea is much more shallow 

 than is usual, we have good reason to believe that, within a 

 period not very remote, the reef has increased considerably 



