)
It should be observed that the sections might have been taken in
any direction through these islands, or through 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.
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