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or later； the coral polypes； like all other things； die； the soft
flesh decays； while the skeleton is left as a stony mass at the
bottom of the sea； where it retains its integrity for a longer or a
shorter time； according as its position affords more or less
protection from the wear and tear of the waves。
The polypes which give rise to the white coral are found； as has
been said； in the seas of all parts of the world； but in the
temperate and cold oceans they are scattered and comparatively
small in size； so that the skeletons of those which die do not
accumulate in any considerable quantity。 But it is otherwise in
the greater part of the ocean which lies in the warmer parts of the
world； comprised within a distance of about eighteen hundred miles
on each side of the equator。 Within the zone thus bounded； by far
the greater part of the ocean is inhabited by coral polypes； which
not only form very strong and large skeletons； but associate
together into great masses； like the thickets and the meadow turf；
or； better still； the accumulations of peat； to which plants give
rise on dry land。 These masses of stony matter； heaped up beneath
the waters of the ocean； become as dangerous to mariners as so much
ordinary rock； and to these； as to the common rock ridges； the
seaman gives the name of 〃reefs。〃
Such coral reefs cover many thousand square miles in the Pacific
and in the Indian Oceans。 There is one reef； or rather great
series of reefs； called the Barrier Reef； which stretches； almost
continuously； for more than eleven hundred miles off the east coast
of Australia。 Multitudes of the islands in the Pacific are either
reefs themselves； or are surrounded by reefs。 The Red Sea is in
many parts almost a maze of such reefs； and they abound no less in
the West Indies； along the coast of Florida； and even as far north
as the Bahama Islands。 But it is a very remarkable circumstance
that； within the area of what we may call the 〃coral zone；〃 there
are no coral reefs upon the west coast of America； nor upon the
west coast of Africa； and it is a general fact that the reefs are
interrupted； or absent； opposite the mouths of great rivers。 The
causes of this apparent caprice in the distribution of coral reefs
are not far to seek。 The polypes which fabricate them require for
their vigorous growth a temperature which must not fall below 68
degrees Fahrenheit all the year round； and this temperature is only
to be found within the distance on each side of the equator which
has been mentioned； or thereabouts。 But even within the coral zone
this degree of warmth is not everywhere to be had。 On the west
coast of America； and on the corresponding coast of Africa； the
currents of cold water from the icy regions which surround the
South Pole set northward； and it appears to be due to their cooling
influence that the sea in these regions is free from the reef
builders。 Again； the coral polypes cannot live in water which is
rendered brackish by floods from the land； or which is perturbed by
mud from the same source； and hence it is that they cease to exist
opposite the mouths of rivers； which damage them in both these
ways。
Such is the general distribution of the reef…building corals； but
there are some very interesting and singular circumstances to be
observed in the conformation of the reefs； when we consider them
individually。 The reefs； in fact； are of three different kinds；
some of them stretch out from the shore； almost like a prolongation
of the beach； covered only by shallow water； and in the case of an
island； surrounding it like a fringe of no considerable breadth。
These are termed 〃fringing reefs。〃 Others are separated by a
channel which may attain a width of many miles； and a depth of
twenty or thirty fathoms or more； from the nearest land； and when
this land is an island； the reef surrounds it like a low wall； and
the sea between the reef and the land is； as it were； a moat inside
this wall。 Such reefs as these are called 〃encircling〃 when they
surround an island； and 〃barrier〃 reefs； when they stretch parallel
with the coast of a continent。 In both these cases there is
ordinary dry land inside the reef； and separated from it only by a
narrower or a wider； a shallower or a deeper； space of sea； which
is called a 〃lagoon；〃 or 〃inner passage。〃 But there is a third kind
of reef； of very common occurrence in the Pacific and Indian
Oceans； which goes by the name of 〃atoll。〃 This is； to all intents
and purposes； an encircling reef； without anything to encircle； or；
in other words； without an island in the middle of its lagoon。 The
atoll has exactly the appearance of a vast； irregularly oval； or
circular； breakwater； enclosing smooth water in its midst。 The
depth of the water in the lagoon rarely exceeds twenty or thirty
fathoms； but； outside the reef； it deepens with great rapidity to
two hundred or three hundred fathoms。 The depth immediately
outside the barrier； or encircling； reefs； may also be very
considerable； but； at the outer edge of a fringing reef； it does
not amount usually to more than twenty or twenty…five fathoms； in
other words； from one hundred and twenty to one hundred and fifty
feet。
Thus； if the water of the ocean should be suddenly drained away； we
should see the atolls rising from the sea…bed like vast truncated
cones； and resembling so many volcanic craters； except that their
sides would be steeper than those of an ordinary volcano。 In the
case of the encircling reefs； the cone； with the enclosed island；
would look like Vesuvius with Monte Nuovo within the old crater of
Somma；＇121＇ while； finally； the island with a fringing reef would
have the appearance of an ordinary hill； or mountain； girded by a vast
parapet； within which would lie a shallow moat。 And the dry bed of
the Pacific might afford grounds for an inhabitant of the moon to
speculate upon the extraordinary subterranean activity to which
these vast and numerous 〃craters〃 bore witness！
When the structure of a fringing reef is investigated； the bottom
of the lagoon is found to be covered with fine whitish mud； which
results from the breaking up of the dead corals。 Upon this muddy
floor there lie； here and there； growing corals； or occasionally
great blocks of dead coral； which have been torn by storms from the
outer edge of the reef； and washed into the lagoon。 Shellfish and
worms of various kinds abound； and fish； some of which prey upon
the coral； sport in the deeper pools。 But the corals which are to
be seen growing in the shallow waters of the lagoon are of a
different kind from those which abound on the outer edge of the
reef； and of which the reef is built up。 Close to the seaward edge
of the reef； over which； even in calm weather； a surf almost always
breaks； the coral rock is encrusted with a thick coat of a singular
vegetable organism； which contains a great deal of limethe so…
called Nullipora。 Be