1836.] DEAD AND SUNKEN KEEFS. 479 



few quite small living points which rise to the surface ; a third 

 and fourth are entirely dead and submerged ; a fifth is a mere 

 wreck, with its structure almost obliterated. It is remarkable 

 that in all these cases, the dead reefs and portions of reef 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 follow s 

 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-resfs red. These latter reefs 

 have been formed whilst the land has been stationary, or, as ap- 

 pears 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 



