dissolve the
carbonate of lime more extensively, and re-deposit it in a crystalline
form. On the beach of the lagoon, where the coral sand is washed into
layers by the action of the waves, its grains become thus fused together
into strata of a limestone, so hard that they ring when struck with a
hammer, and inclined at a gentle angle, corresponding with that of the
surface of the beach. The hard parts of the many animals which live upon
the reef become imbedded in this coral limestone, so that a block may
be full of shells of bivalves and univalves, or of sea urchins; and even
sometimes encloses the eggs of turtles in a state of petrification.
The active and vigorous growth of the reef goes on only at the seaward
margins, where the polypes are exposed to the wash of the surf, and
are thereby provided with an abundant supply of air and of food.
The interior portion of the reef may be regarded as almost wholly an
accumulation of dead skeletons. Where a river comes down from the land
there is a break in the reef, for the reasons which have been already
mentioned.

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