XX 
FRINGING - REEFS 
501 
why the corals should have sprung up, like a wall, from the 
extreme outer margin of the ledge, often leaving a broad space 
of water within, too deep for the growth of corals. The 
accumulation of a wide bank of sediment all round these islands, 
and generally widest where the included islands are smallest, is 
highly improbable, considering their exposed positions in the 
central and deepest parts of the ocean. In the case of the 
barrier-reef of New Caledonia, which extends for 150 miles 
beyond the northern point of the island, in the same straight 
line with which it fronts the west coast, it is hardly possible to 
believe that a bank of sediment could thus have been straightly 
deposited in front of a lofty island, and so far beyond its 
termination in the open sea. Finally, if we look to other 
oceanic islands of about the same height and of similar geological 
constitution, but not encircled by coral-reefs, we may in vain 
search for so trifling a circumambient depth as 30 fathoms, 
except quite near to their shores ; for usually land that rises 
abruptly out of water, as do most of the encircled and non- 
encircled oceanic islands, plunges abruptly under it. O11 what 
then, I repeat, are these barrier-reefs based ? Why, with their 
wide and deep moat-like channels, do they stand so far from the 
included land ? We shall soon see how easily these difficulties 
disappear. 
We come now to our third class of Fringing-reefs, which 
will require a very short notice. Where the land slopes 
abruptly under water, these reefs are only a few yards in width, 
forming a mere ribbon or fringe round the shores : where the 
land slopes gently under the water the reef extends farther, 
sometimes even as much as a mile from the land ; but in such 
cases the soundings outside the reef always show that the 
submarine prolongation of the land is gently inclined. In 
fact the reefs extend only to that distance from the shore at 
which a foundation within the requisite depth from 20 to 30 
fathoms is found. As far as the actual reef is concerned, there 
is no essential difference between it and that forming a barrier 
or an atoll: it is, however, generally of less width, and 
consequently few islets have been formed on it. From the 
corals growing more vigorously on the outside, and from the 
noxious effect of the sediment washed inwards, the outer edge 
of the reef is the highest part, and between it and the land 
