154 
THEORY OF THE FORMATION 
Ch. V. 
barrier-reefs are supposed to have gone on subsiding 
for a long period, yet that their lagoons and lagoon- 
channels have only rarely come to exceed 40 and 
never 60 fathoms in depth. But if our theory is worth 
consideration, we already admit that the rate of sub- 
sidence has not ordinarily exceeded that of the upward 
growth of the massive corals which live on the margins 
of the reefs, so that we have only further to suppose 
that the rate has never exceeded that at which lagoons 
and lagoon- channels are filled up by the growth of the 
delicate corals which live there, and by the accumula- 
tion of sediment. As the filling-up process, in the case 
of barrier-reefs lying far from the land, and of the larger 
atolls, must be an extremely slow one, we are led to 
conclude that the subsiding movement has always been 
equally slow. And this conclusion accords well with 
what is known of the rate of recent movements of 
elevation. 
It has, I think, been shown in this chapter, that 
subsidence explains both the normal structure and 
the less regular forms of those two great classes of 
reefs which have justly excited the astonishment of all 
the naturalists who have sailed through the Pacific and 
Indian Oceans. The necessity, also, that a foundation 
should have existed at the proper depth for the growth 
of the corals over certain large areas, almost compels 
us to accept this theory. But further to test its truth, 
a crowd of questions may be asked. Bo the different 
kinds of reefs which have been produced by the 
same kind of movement, generally lie within the 
