THE VOYAGE OF THE BEAGLE 



505 



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 car- 

 ried 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 sub- 

 sidence, 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 exist- 

 ence, 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 sta- 

 tionary, 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 circum- 

 stances 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 



