the same. Now, bearing in mind that reef-building coral cannot live at
a greater depth than from 20 to 30 fathoms, and that the scale is so
small that the plummets on the right hand show a depth of 200 fathoms,
on what are these barrier-reefs based? Are we to suppose that each
island is surrounded by a collar-like submarine ledge of rock, or by a
great bank of sediment, ending abruptly where the reef ends?
If the sea had formerly eaten deeply into the islands, before they were
protected by the reefs, thus having left a shallow ledge round them
under water, the present shores would have been invariably bounded by
great precipices, but this is most rarely the case. Moreover, on this
notion, it is not possible to explain 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 islands, 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. On 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 further, 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
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