194 
RECAPITULATION. 
Cn. VI. 
at a much greater distance from the land, and in 
the presence of a deep lagoon-like space within the 
reef. In the fourth chapter the growing powers of the 
reef-constructing polypifers were discussed ; and it was 
shown that they cannot flourish beneath a very limited 
depth. In accordance with this limit, there is no diffi- 
culty respecting the foundation on which a fringing- 
reef is based ; whereas, with harrier-reefs and atolls, 
there is the greatest difficulty on this head ; — in bar- 
rier-reefs from the improbability of rock or hanks of 
sediment having extended, in every instance, so far 
seaward within the required depth ; — and in atolls, 
from the immensity of the spaces over which they are 
interspersed, and the apparent necessity for believing 
that they are all based on mountain-summits, which, 
although rising very near to the surface of the sea, in 
no one instance rise above it. To escape this latter 
admission, which implies the existence of submarine 
chains of mountains of almost exactly the same height 
extending over many thousand square miles, there is 
hut one alternative ; namely, the prolonged subsidence 
of the foundations on which the atolls first became 
attached, together with the upward growth of the 
reef-constructing corals. On this view every difficulty 
vanishes : fringing-reefs are thus easily converted into 
barrier-reefs ; and barrier-reefs into atolls, as soon as 
the last pinnacle of land sinks beneath the surface of 
the sea. 
The wall-like structure on the inner sides of atolls 
and barrier-reefs— the basin or ring-like shape of the 
