The curious practical result follows from this structure,
that the lagoons to these reefs really form admirable harbours, if
a ship can only get inside them. But the main difference between the
encircling reefs and the atolls, on the one hand, and the fringing reefs
on the other, lies in the fact of the much greater depth of water on the
seaward faces of the former. As a consequence of this fact, the whole
of this face is not, as it is in the case of the fringing reef, covered
with living coral polypes. For, as we have seen, these polypes cannot
live at a greater depth than about twenty-five fathoms; and actual
observation has shown that while, down to this depth, the sounding-lead
will bring up branches of live coral from the outer wall of such a reef,
at a greater depth it fetches to the surface nothing but dead coral and
coral sand. We must, therefore, picture to ourselves an atoll, or an
encircling reef, as fringed for one hundred feet, or more, from its
summit, with coral polypes busily engaged in fabricating coral; while,
below this comparatively narrow belt, its surface is a bare and smooth
expanse of coral sand, supported upon and within a core of coral
limestone. Thus, if the bed of the Pacific were suddenly laid bare, as
was just now supposed, the appearance of the reef-mountains would be
exactly the reverse of that presented by many high mountains on land.
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