CORAL-REEFS. 35 



margin of the reef grew either upwards, or as I believe 

 outwards, to its present distance from the beach. 



Whether this view be correct or not, the above facts are 

 worthy of attention, as showing how severe a struggle is in 

 progress on these low coral formations between the two 

 nicely balanced powers of land and water. With respect to 

 the future state of Keeling atoll, if left undisturbed, we can 

 see that the islets may still extend in length ; but as they 

 cannot resist the surf until broken by rolling over a wide 

 space, their increase in breadth must depend on the increasing 

 breadth of the reef; and this must be limited by the steep- 

 ness of the submarine flanks, which can be added to only 

 by sediment derived from the wear and tear of the coral. 

 From the rapid growth of the coral in the channel cut for 

 the schooner, and from the several agents at work in pro- 

 ducing fine sediment, it might be thought that the lagoon 

 would necessarily become quickly filled up. Some of this 

 sediment, however, is transported into the open sea, as 

 appears from the soundings off the mouth of the lagoon, 

 instead of being deposited within it. The deposition, 

 moreover, of sediment checks the growth of coral-reefs, so 

 that these two agencies cannot act together with full effect 

 in filling it up. We know so little of the habits of the 

 many different species of corals, which form the lagoon- 

 reefs, that we' have no more reasons for supposing that their 

 whole surface would grow up as quickly as the coral did in 

 the schooner-channel, than for supposing that the whole 

 surface of a peat-moss would increase as quickly as parts are 

 known to do in holes, where the peat has been cut away. 

 These agencies, nevertheless, tend to fill up the lagoon ; but 

 in proportion as it becomes shallower, so must the polypifers 

 be subject to many injurious agencies, such as impure water 

 and loss of food. For instance, Mr. Liesk informed me, 



