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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 thick； and all full of marine remains； occur in
various parts of the earth's surface； and prove； beyond a doubt；
that when these beds were formed； that portion of the sea…bottom
which they then occupied underwent a slow and gradual depression to
a distance which cannot have been less than the thickness of those
beds； and may have been very much greater。 In supposing；
therefore； that the great areas of the Pacific and of the Indian
Ocean； over which atolls and encircling reefs are found scattered；
have undergone a depression of some hundreds； or； it may be；
thousands of feet； Mr。 Darwin made a supposition which had nothing
forced or improbable； but was entirely in accordance with what we
know to have taken place over similarly extensive areas； in other
periods of the world's history。 But Mr。 Darwin subjected his
hypothesis to an ingenious indirect test。 If his view be correct；
it is clear that neither atolls； nor encircling reefs； should be
found in those portions of the ocean in which we have reason to
believe； on independent grounds； that the sea…bottom has long been
either stationary； or slowly rising。 Now it is known that； as a
general rule； the level of the land is either stationary； or is
undergoing a slow upheaval； in the neighborhood of active
volcanoes； and； therefore； neither atolls nor encircling reefs
ought to be found in regions in which volcanoes are numerous and
active。 And this turns out to be the case。 Appended to Mr。
Darwin's great work on coral reefs； there is a map on which atolls
and encircling reefs are indicated by one colour； fringing reefs by
another； and active volcanoes by a third。 And it is at once
obvious that the lines of active volcanoes lie around the margins
of the areas occupied by the atolls and the encircling reefs。 It
is exactly as if the upheaving volcanic agencies had lifted up the
edges of these great areas； while their centres had undergone a
corresponding depression。 An atoll area may； in short； be pictured
as a kind of basin； the margins of which have been pushed up by the
subterranean forces； to which the craters of the volcanoes have； at
intervals； given vent。
Thus we must imagine the area of the Pacific now covered by the
Polynesian Archipelago； as having been； at some former time；
occupied by large islands； or； may be； by a great continent； with
the ordinarily diversified surface of plain； and hill； and mountain
chain。 The shores of this great land were doubtless fringed by
coral reefs； and； as it slowly underwent depression； the hilly
regions； converted into islands； became； at first； surrounded by
fringing reefs； and then； as depression went on； these became
converted into encircling reefs； and these； finally； into atolls；
until a maze of reefs and coral…girdled islets took the place of
the original land masses。
Thus the atolls and the encircling reefs furnish us with clear；
though indirect； evidence of changes in the physical geography of
large parts of the earth's surface； and even； as my lamented
friend； the late Professor Jukes；＇124＇ has suggested； give us
indications of the manner in which some of the most puzzling facts
connected with the distribution of animals have been brought about。
For example； Australia and New Guinea are separated by Torres
Straits； a broad belt of sea one hundred or one hundred and twenty
miles wide。 Nevertheless； there is in many respects a curious
resemblance between the land animals which inhabit New Guinea and
the land animals which inhabit Australia。 But； at the same time；
the marine shellfish which are found in the shallow waters of the
shores of New Guinea are quite different from those which are met
with upon the coasts of Australia。 Now； the eastern end of Torres
Straits is full of atolls； which； in fact； form the northern
termination of the Great Barrier Reef which skirts the eastern
coast of Australia。 It follows； therefore； that the eastern end of
Torres Straits is an area of depression； and it is very possible；
and on many grounds highly probable； that； in former times；
Australia and New Guinea were directly connected together； and that
Torres Straits did not exist。 If this were the case； the existence
of cassowaries and of marsupial quadrupeds； both in New Guinea and
in Australia； becomes intelligible； while the difference between
the littoral molluscs of the north and the south shores of Torres
Straits is readily explained by the great probability that； when
the depression in question took place； and what was； at first； an
arm of the sea became converted into a strait separating Australia
from New Guinea； the northern shore of this new sea became tenanted
with marine animals from the north； while the southern shore was
peopled by immigrants from the already existing marine Australian
fauna。
Inasmuch as the growth of the reef depends upon that of successive
generations of coral polypes； and as each generation takes a
certain time to grow to its full size； and can only separate its
calcareous skeleton from the water in which it lives at a certain
rate； it is clear that the reefs are records not only of changes in
physical geography； but of the lapse of time。 It is by no means
easy； however； to estimate the exact value of reef chronology； and
the attempts which have been made to determine the rate at which a
reef grows vertically have yielded anything but precise results。 A
cautious writer； Mr。 Dana；＇125＇ whose extensive study of corals and
coral reefs makes him an eminently competent judge； states his
conclusion in the following terms：
〃The rate of growth of the common branching madrepore is not over
one and a half inches a year。 As the branches are open； this would
not be equivalent to more than half an inch in height of solid
coral for the whole surface covered by the madrepore； and； as they
are also porous； to not over three…eighths of an inch of solid
limestone。 But a coral plantation has large bare patches without
corals； and the coral sands are widely distributed by currents；
part of them to depths over one hundred feet where there are no
living corals； not more than one…sixth of the surface of a reef
region is； in fact； covered with growing species。 This reduces the
three…eighths to ONE…SIXTEENTH。 Shells and other organic relics
may contribute one…fourth as much as corals。 At the outside； the
average upward increase of the whole reef…ground per year would not
exceed ONE…EIGHTH of an inch。
〃Now some reefs are at least two thousand feet thick； which at one…
eighth of an inch a year； corresponds to one hundred and ninety…two
thousand years。〃*
* Dana； Manual of Geology； p。 591。
Halve； or quarter； this estimate if you will； in order to be
certain of erring upon the right side； and still there remains a
prodigious period during which the ancestors of existing coral
polypes have been undisturbedly at work； and during which；
therefore； the climatal conditions over the coral area must have
been much what they are now。
And all this lapse of time has occurred within the most recent
period of the history of the earth。 The remains of reefs formed by
coral polypes of different kinds from those which exist now； enter
largely into the composition of the limestones of the Jurassic
period；＇126＇ and still more widely different coral polypes have
contributed their quota to the vast thickness of the carboniferous
and Devonian strata。 Then as regards the latter group of rocks in
America； the high authority already quoted tells us：
〃The Upper Helderberg period is eminently the coral reef period of
the palaeozoic ages。 Many of the rocks abound in coral； and are as
truly coral reefs as the modern reefs of the Pacific。 The corals
are sometimes standing on the rocks in the position they had when
growing： others are lying in fragments； as they were bro