XX DISTRIBUTION OF CORAL-REEFS 509 



lie at nearly the same depth, namely, from six to eight fathoms 

 beneath the surface, as if they had been carried down by one 

 uniform movement. One of these " half-drowned atolls," so 

 called by Capt. Moresby (to whom I am indebted for much 

 invaluable information), is of vast size, namely, ninety nautical 

 miles across in one direction, and seventy miles in another line ; 

 and is in many respects eminently curious. As by our theory 

 it follows that new atolls will generally be formed in each new 

 area of subsidence, two weighty objections might have been 

 raised, namely, that atolls must be increasing indefinitely in 

 number ; and secondly, that in old areas of subsidence each 

 separate atoll must be increasing indefinitely in thickness, if 

 proofs of their occasional destruction could not have been 

 adduced. Thus have we traced the history of these great rings 

 of coral -rock, from their first origin through their normal 

 changes, and through the occasional accidents of their existence, 

 to their death and final obliteration. 



In my volume on Coral Foniiatioiis I have published a 

 map, in which I have coloured all the atolls dark blue, the 

 barrier-reefs pale blue, and the fringing- reefs red. These 

 latter reefs have been formed whilst the land has been 

 stationary, or, as appears from the frequent presence of 

 upraised organic remains, whilst it has been slowly rising : 

 atolls and barrier- reefs, on the other hand, have grown up 

 during the directly opposite movement of subsidence, which 

 movement must have been very gradual, and in the case of 

 atolls so vast in amount as to have buried every mountain- 

 summit over wide ocean -spaces. Now in this map we see 

 that the reefs tinted pale and dark blue, which have been 

 produced by the same order of movement, as a general rule 

 manifestly stand near each other. Again we see that the 

 areas with the two blue tints are of wide extent ; and that 

 they lie separate from extensive lines of coast coloured red, 

 both of which circumstances might naturally have been inferred, 

 on the theory of the nature of the reefs having been governed 

 by the nature of the earth's movement. It deserves notice, 

 that in more than one instance where single red and blue 

 circles approach near each other, I can show that there have 

 been oscillations of level ; for in such cases the red or fringed 



