southwest side: while in the southern hemisphere,
where the trade-winds blow from the southeast, the opening lies to the
northwest. 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.
For these are white with snow at the top, while their bases are clothed
with an abundant and gaudily-coloured vegetation. But the coral cones
would look grey and barren below, while their summits would be gay with
a richly-coloured parterre of flowerlike coral polypes.

The practical difficulties of sounding upon, and of bringing up portions
of, the seaward face of an atoll or of an encircling reef, are so great,
in consequence of the constant and dangerous swell which sets towards
it, that no exact information concerning the depth to which the reefs
are composed of coral has yet been obtained. There is no reason to
doubt, however, that the reef-cone has the same structure from its
summit to its base, and that its sea-wall is throughout mainly composed
of dead coral.

And now arises a serious difficulty. If the coral polypes cannot live at
a greater depth than one hundred or one hundred and fifty feet, how can
they have built