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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If； as we have supposed； the sea could be suddenly withdrawn from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 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。
The curious practical result follows from this structure； that the
lagoons to 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 one hundred 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
flowerlike coral polypes。
The practical difficulties of sounding upon； and of bringing up
portions of； the seaward face of an atoll or of an encircling reef；
are so great； in consequence of the constant and dangerous swell
which sets towards it； that no exact information concerning the
depth to which the reefs are composed of coral has yet been
obtained。 There is no reason to doubt； however； that the reef…cone
has the same structure from its summit to its base； and that its
sea…wall is throughout mainly composed of dead coral。
And now arises a serious difficulty。 If the coral polypes cannot
live at a greater depth than one hundred or one hundred and fifty
feet； how can they have built up the base of the reef…cone； which
may be two thousand feet； or more； below the surface of the sea？
In order to get over this objection；