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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。 Beyond this； in the part of the edge of the reef
which is always covered by the breaking waves； the living； true；
reef…polypes make their appearance； and； in different forms； coat
the steep seaward face of the reef to a depth of one hundred or
even one hundred and fifty feet。 Beyond this depth the sounding…
lead rests； not upon the wall…like face of the reef； but on the
ordinary shelving sea…bottom。 And the distance to which a fringing
reef extends from the land corresponds with that at which the sea
has a depth of twenty or five…and…twenty fathoms。
If； as we have supposed； the sea could be suddenly withdrawn from
around an island provided with a fringing reef； such as the
Mauritius；＇122＇ the reef would present the aspect of a terrace；
its seaward face； one hundred feet or more high； blooming with the
animal flowers of the coral； while its surface would be hollowed
out into a shallow and irregular moat…like excavation。
The coral mud； which occupies the bottom of the lagoon； and with
which all the interstices of the coral skeletons which accumulate
to form the reef are filled up； does not proceed from the washing
action of the waves alone； innumerable fishes； and other creatures
which prey upon the coral； add a very important contribution of
finely…triturated calcareous matter； and the corals and mud
becoming incorporated together； gradually harden and give rise to a
sort of limestone rock； which may vary a good deal in texture。
Sometimes it remains friable and chalky； but； more often； the
infiltration of water； charged with carbonic acid； dissolves some
of the calcareous matter； and deposits it elsewhere in the
interstices of the nascent rock； thus glueing and cementing the
particles together into a hard mass； or it may even 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 a