umulation in the encircling reefs and
in the atolls is essentially the same as in the fringing reef. But, in
addition to the differences of depth inside and out, they present
some other peculiarities. These reefs, and especially the atolls, are
usually interrupted at one part of their circumference, and this part
is always situated on the leeward side of the reef, or that which is
the more sheltered side. Now, as all these reefs are situated within
the region in which the trade-winds prevail, it follows that, on the
north side of the equator, where the trade-wind is a north-easterly
wind, the opening of the reef is on the south-west side: while in the
southern hemisphere, where the trade-winds blow from the south-east,
the opening lies to the north-west. The curious practical result
follows from this structure, that the lagoons of 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 100 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 flower-like coral polypes.

The practical difficulties of sounding upon,