Many Other Encircled Islands, And The General Features Would
Have Been 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.
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