The origin and mode of formation of a fringing reef, such as that just
described, are plain enough. The embryos of the coral polypes have fixed
themselves upon the submerged shore of the island, as far out as they
could live, namely, to a depth of twenty or twenty-five fathoms. One
generation has succeeded another, building itself up upon the dead
skeletons of its predecessor. The mass has been consolidated by
the infiltration of coral mud, and hardened by partial solution and
redeposition, until a great rampart of coral rock one hundred or one
hundred and fifty feet high on its seaward face has been formed all
round the island, with only such gaps as result from the outflow of
rivers, in the place of sally-ports.
The structure of the rocky accumulation 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 tradewinds prevail, it follows that, on the north side of
the equator, where the trade-wind is a northeasterly wind, the opening
of the reef is on the southwest side: while in the southern hemisphere,
where the trade-winds blow from the southeast, the opening lies to the
northwest.
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