100 
RATE OF GROWTH. 
Ch. IV 
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 Nullipor® cannot increase outwards 
without encroaching on the Porites and Millepora 
complanata, as is now taking place; nor these latter 
without encroaching on the strongly branched Madre- 
pora, the Millepora alcicornis, and some Astraeas ; 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 
‘ that the island has been produced by accessions from 
the sea, and is in a state of increase.’ The land is 
frontf d by a coral-reef, and from the manner in which 
1 Cook’s Third Voyage, book iii. ch. x. 
