autobiography and selected essays-第35章
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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; it was at one time supposed
that the reef…building polypes had settled upon the summits of a
chain of submarine mountains。 But what is there in physical
geography to justify the assumption of the existence of a chain of
mountains stretching for one thousand miles or more; and so nearly
of the same height; that none should rise above the level of the
sea; nor fall one hundred and fifty feet below that level?
How; again; on this hypothesis; are atolls to be accounted for;
unless; as some have done; we take refuge in the wild supposition
that every atoll corresponds with the crater of a submarine
volcano? And what explanation does it afford of the fact that; in
some parts of the ocean; only atolls and encircling reefs occur;
while others present none but fringing reefs?
These and other puzzling facts remained insoluble until the
publication; in the year 1840; of Mr。 Darwin's famous work on coral
reefs;'123' in which a key was given to all the difficult problems
connected with the subject; and every difficulty was shown to be
capable of solution by deductive reasoning from a happy combination
of certain well…established geological and biological truths。 Mr。
Darwin; in fact; showed that; so long as the level of the sea
remains unaltered in any area in which coral reefs are being
formed; or if the level of the sea relatively to that of the land
is falling; the only reefs which can be formed are fringing reefs。
While if; on the contrary; the level of the sea is rising
relatively to that of the land; at a rate not faster than that at
which the upward growth of the coral can keep pace with it; the
reef will gradually pass from the condition of a fringing; into
that of an encircling or barrier reef。 And; finally; that if the
relative level of the sea rise so much that the encircled land is
completely submerged; the reef must necessarily pass into the
condition of an atoll。
For; suppose the relative level of the sea to remain stationary;
after a fringing reef has reached that distance from the land at
which the depth of water amounts to one hundred and fifty feet。
Then the reef cannot extend seaward by the migration of coral
germs; because these coral germs would find the bottom of the sea
to be too deep for them to live in。 And the only manner in which
the reef could extend outwards; would be by the gradual
accumulation; at the foot of its seaward face; of a talus of coral
fragments torn off by the violence of the waves; which talus might;
in course of time; become high enough to bring its upper surface
within the limits of coral growth; and in that manner provide a
sort of factitious sea…bottom upon which the coral embryos might
perch。 If; on the other hand; the level of the sea were slowly and
gradually lowered; it is clear that the parts of its bottom
originally beyond the limit of coral growth would gradually be
brought within the required distance of the surface; and thus the
reef might be indefinitely extended。 But this process would give
rise neither to an encircling reef nor to an atoll; but to a broad
belt of upheaved coral rock; increasing the dimensions of the dry
land; and continuous seawards with the fresh fringing reef。
Suppose; however; that the sea…level rose instead of falling; at
the same slow and gradual rate at which we know it to be rising in
some parts of the world;not more; in fact; than a few inches; or;
at most; a foot or two; in a hundred years。 Then; while the reef
would be unable to extend itself seaward; the sea…bottom outside it
being gradually more and more removed from the depth at which the
life of the coral polypes is possible; it would be able to grow
upwards as fast as the sea rose。 But the growth would take place
almost exclusively around the circumference of the reef; this being
the only region in which the coral polypes would find the
conditions favourable for their existence。 The bottom of the
lagoon would be raised; in the main; only by the coral debris and
coral mud; formed in the manner already described; consequently;
the margins of the reef would rise faster than the bottom; or; in
other words; the lagoon would constantly become deeper。 And; at
the same time; it would gradually increase in breadth; as the
rising sea; covering more of the land; would occupy a wider space
between the edge of the reef and what remained of the land。 Thus
the rising sea would eventually convert a large island with a
fringing reef into a small island surrounded by an encircling reef。
And it will be obvious that when the rising of the sea has gone so
far as completely to cover the highest points of the island; the
reef will have passed into the condition of an atoll。
But how is it possible that the relative level of the land and sea
should be altered to this extent? Clearly; only in one of two
ways: either the sea must have risen over those areas which are now
covered by atolls and encircling reefs; or; the land upon which the
sea rests must have been depressed to a corresponding extent。
If the sea has risen; its rise must have taken place over the whole
world simultaneously; and it must have risen to the same height
over all parts of the coral zone。 Grounds have been shown for the
belief that the general level of the sea may have been different at
different times; it has been suggested; for example; that the
accumulation of ice about the poles during one of the cold periods
of the earth's history necessarily implies a diminution in the
volume of the sea proportioned to the amount of its water thus
permanently locked up in the Arctic and Antarctic ice…cellars;
while; in the warm periods; the greater or less disappearance of
the polar ice…cap implies a corresponding addition of water to the
ocean。 And no doubt this reasoning must be admitted to be sound in
principle; though it is very hard to say what practical effect the
additions and subtractions thus made have had on the level of the
ocean; inasmuch as such additions and subtractions might be either
intensified or nullified; by contemporaneous changes in the level
of the land。 And no one has yet shown that any such great melting
of polar ice; and consequent raising of the level of the water of
the ocean; has taken place since the existing atolls began to be
formed。
In the absence of any evidence that the sea has ever risen to the
extent required to give rise to the encircling reefs and the
atolls; Mr。 Darwin adopted the opposite hypothesis; viz。; that the
land has undergone extensive and slow depression in those
localities in which these structures exist。
It seems; at first; a startling paradox; to suppose that the land
is less fixed than the sea; but that such is the case is the
uniform testimony of geology。 Beds of sandstone or limestone;
thousands of feet thic