Ch. V. 
OF CORAL-REEFS. 
137 
atoll, except a diminution in its size, from the reef not 
growing vertically upwards. I may here observe, that 
a bank either of rock or of hardened sediment, level 
with the surface of the sea and fringed with living 
coral, would be immediately converted by subsidence 
into an atoll, without passing, as in the case of a reef 
fringing the shore of an island, through the inter- 
mediate form of a barrier-reef. As before remarked, if 
such a bank lay a few fathoms submerged, the simple 
growth of the coral, without the aid of subsidence, would 
produce a structure scarcely to be distinguished from a 
true atoll; for the corals on the outer margin, from being 
freely exposed to the open sea, would grow vigorously 
and tend to form a continuous ring, w'hilst the growth 
of the less massive kinds on the central expanse, would 
be checked by the sediment formed there, and by that 
washed inwards by the breakers ; and as the space be- 
came shallower, their growth would also be checked by 
the impurities of the water, and probably by the small 
amount of food brought to them by the enfeebled cur- 
rents. The subsidence of a reef based on a bank of this 
kind, would give depth to the central expanse or lagoon, 
steepness to the flanks, and through the free growth of 
the coral, symmetry to the whole outline ; but, as we 
have seen, the larger groups of atolls in the Pacific and 
Indian Oceans cannot have been formed on banks of 
this nature. 
If, instead of an island, as in the diagram, the shore 
of a continent fringed by a reef were to subside, a great 
barrier-reef like that on the N.E. coast of Australia, 
