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 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 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 
