Ch. V. 
OF CORAL-REEFS. 
127 
impossible ; for the upheaval and subsequent abrasion 
of an island would leave a flat disk, which might become 
coated with coral, but not a deeply concave surface ; 
moreover, we should expect to see, at least in some 
parts, the rock of the foundation brought to the surface. 
If, then, the foundations of the many atolls were not 
uplifted into the requisite position, they must of neces- 
sity have subsided into it ; and this at once solves every 
difficulty, 1 for we may safely infer from the facts given 
in the last chapter, that during a gradual subsidence 
the corals would be favourably circumstanced for build- 
ing up their solid frameworks and reaching the surface, 
as island after island slowly disappeared. Thus areas 
of immense extent in the central and most profound 
1 The additional difficulty on the crater hypothesis before alluded 
to, will now be evident ; for on this view the volcanic action must 
be supposed to have formed within the areas specified a vast number 
of craters, all rising within a few fathoms of the surface, and not one 
above it. The supposition that the craters were at different times 
upraised above the surface, and were there abraded by the surf and 
subsequently coated by corals, is subjected to nearly the same ob- 
jections with those given at the top of the page ; but I consider 
it superfluous to detail all the arguments opposed to such a notion. 
Chamisso’s theory, from assuming the existence of so many banks, 
all lying at the proper depth beneath the water, is also vitally de- 
fective. The same observation applies to an hypothesis of Lieut. 
Nelson’s (Geolog. Trans, vol. v. p. 122), who supposes that the ring- 
formed structure is caused by a greater number of germs of corals 
becoming attached to the declivity, than to the central plateau of a 
submarine bank ; it likewise applies to the notion formerly enter- 
tained (Forster’s Observ. p. 151), that lagoon-islands owe their pecu- 
liar form to the instinctive tendencies of the polypifers. According 
to this latter view, the corals on the outer margin of the reef in- 
stinctively oppose themselves to the surf in order to afford protection 
to corals living in the lagoon which belong to other genera and to 
other families 1 
