100 RATE OF GROWTH. Ch. IV. 



complanata, as is now taking place ; nor these latter 

 without encroaching' on the strongly branched Madre- 

 pora, the Millepora alcicomis, 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 in- 

 creased considerably in width. The land has the 

 extraordinary breadth of three miles ; it consists of 

 parallel ridges of shells and broken corals, which 

 furnish 'an incontestable proof,' as observed by Cook, 1 

 1 that the island has been produced by accessions from 

 the sea, and is in a state of increase.' The land is 

 fronted by a coral-reef, and from the manner in which 

 islets are known to be formed, we may feel confident 

 that the reef was not three miles wide when the first, 

 or most inward ridge, was thrown up ; and, there- 

 fore, we must conclude that the reef has grown out- 

 wards during the accumulation of the successive ridges. 

 Here then, a Wall of coral-rock of very considerable 

 breadth has been formed by the outward growth of 

 the living margin, within a period, during which 

 ridges of shells and corals, lying on the bare surface, 

 have not decayed. There can be little doubt, from the 

 account given by Captain Beechey, that Matilda atoll in 

 the Low Archipelago has been converted in the space 



1 Cook's Third Voyage, book iii. ch. x. 



