DEAD AND .SUNKEN REEFS. [CHAP. xx. 



surface, as if they had been carried down by one uniform move- 

 ment. One of these "half-drowned atolls," so called by Capt. 

 Moresby (to whom I am indebted for much invaluable informa- 

 tion), 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 Formations " 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 natural ly 

 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 circles 

 consist of atolls, originally by our theory formed during subsidence, 

 but subsequently upheaved ; and on the other hand, some of the 

 pale-blue or encircled islands are composed of coral-rock, which 

 must have been uplifted to its present height before that sub- 



