1836.] DEAD AND SUNKEN REFS, 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 directitn, and seventy miles ih 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. is 
In my volume on ‘ Coral Formations’ I have published a map, 
in which I have coloured all the atolls dark-blue, the barrier- 
reefs paile-blue, and the fringing-reefs 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 
