Ch. II. 
BAKEIER BEEFS. 
67 
the land, leaving a deep and broad moat within, 
remains altogether unexplained. A supposition of the 
same nature and appearing at first more probable, is, 
that the reefs have risen from banks of sediment, 
which had accumulated round the shore previously to 
the growth of the coral ; but the extension of a bank 
to the same distance round an unbroken coast, and 
in front of deep arms of the sea (as in Eaiatea, see 
Plate II., fig. 3), which penetrate nearly to the heart of 
some encircled islands, is exceedingly improbable. And 
why, again, should the reef, in some cases steep on both 
sides like a wall, spring up at a distance of two, three, or 
more miles from the shore, leaving a channel often be- 
tween 200 and 300 feet deep — a depth which, we have 
good reason to believe, is too great for the growth of 
coral ? The existence, also, of this same channel pre- 
cludes the idea of the reef having grown outwards, on a 
foundation slowly formed by the accumulation of its own 
detritus and sediment. Nor, again, can it be asserted 
that the reef-building corals will not grow, excepting at 
a great distance from the land ; for, as we shall soon see, 
there is a whole class of reefs which take their name 
from growing (especially where the sea is deep) closely 
attached to the shore. At New Caledonia (see Plate 
II., fig. 5), the reefs which run in front of the west coast 
are prolonged in the same line for 150 miles beyond the 
northern extremity of the island, and this shows that 
some explanation, quite different from any one of those 
just suggested is requisite. If the island had been 
originally prolonged to this distance, and if the northern 
