CORAL AND CORAL REEFS.*

([Footnote] *A Lecture delivered in Manchester, November 4th, 1870.)

The subject upon which I wish to address you to-night is the structure

and origin of Coral and Coral Reefs. Under the head of "coral" there are

included two very different things; one of them is that substance which

I imagine a great number of us have champed when we were very much

younger than we are now,--the common red coral, which is used so much,

as you know, for the edification and the delectation of children of

tender years, and is also employed for the purposes of ornament for

those who are much older, and as some think might know better. The other

kind of coral is a very different substance; it may for distinction's

sake be called the white coral; it is a material which most assuredly

not the hardest-hearted of baby farmers would give to a baby to chew,

and it is a substance which is to be seen only in the cabinets of

curious persons, or in museums, or, may be, over the mantelpieces of

sea-faring men. But although the red coral, as I have mentioned to you,

has access to the very best society; and although the white coral is

comparatively a despised product, yet in this, as in many other cases,

the humbler thing is in reality the greater; the amount of work which is

done in the world by the white coral being absolutely infinite compared

with that effected by its delicate and pampered namesake. Each of these

substances, the white coral and the red, however, has a relationship to

the other. They are, in a zoological sense, cousins, each of them being

formed by the same kind of animals in what is substantially the same

way. Each of these bodies is, in fact, the hard skeleton of a very

curious and a very simple animal, more comparable to the bones of such

animals as ourselves than to the shells of oysters or creatures of that

kind; for it is the hardening of the internal tissue of the creature, of

its internal substance, by the deposit in the body of a material which

is exceedingly common, not only in fresh but in sea water, and which

is specially abundant in those waters which we know as "hard,"

those waters, for example, which leave a "fur" upon the bottom of a

tea-kettle. This "fur" is carbonate of lime, the same sort of substance

as limestone and chalk. That material is contained in solution in sea

water, and it is out of the sea water in which these coral creatures

live that they get the lime which is needed for the forming of their

hard skeleton.

But now what manner of creatures are these which form these hard

skeletons? I dare say that in these days of keeping aquaria, of

locomotion to the sea-side, most of those whom I am addressing may have

seen one of those creatures which used to be known as the "sea anemone,"

receiving that name on account of its general resemblance, in a rough

sort of way, to the flower which is known as the "anemone"; but being

a thing which lives in the sea, it was qualified as the "sea anemone."

Well, then, you must suppose a body shaped like a short cylinder, the

top cut off, and in the top a hole rather oval than round. All round

this aperture, which is the mouth, imagine that there are placed a

number of feelers forming a circle. The cavity of the mouth leads into

a sort of stomach, which is very unlike those of the higher animals,

in the circumstance that it opens at the lower end into a cavity of the

body, and all the digested matter, converted into nourishment, is thus

distributed through the rest of the body. That is the general structure

of one of these sea anemones. If you touch it it contracts immediately

into a heap. It looks at first quite like a flower in the sea, but if

you touch it you find that it exhibits all the peculiarities of a living

animal; and if anything which can serve as its prey comes near its

tentacles, it closes them round it and sucks the material into its

stomach and there digests it and turns it to the account of its own

body.

These creatures are very voracious, and not at all particular what they

seize; and sometimes it may be that they lay hold of a shellfish which

is far too big to be packed into that interior cavity, and, of course,

in any ordinary animal a proceeding of this kind would give rise to a

very severe fit of indigestion. But this is by no means the case in the

sea anemone, because when digestive difficulties of this kind arise he

gets out of them by splitting himself in two; and then each half builds

itself up into a fresh creature, and you have two polypes where there

was previously one, and the bone which stuck in the way lying between

them! Not only can these creatures multiply in this fashion, but they

can multiply by buds. A bud will grow out of the side of the body (I am

not speaking of the common sea anemone, but of allied creatures) just

like the bud of a plant, and that will fashion itself into a creature

just like the parent. There are some of them in which these buds remain

connected together, and you will soon see what would be the result of

that. If I make a bud grow out here, and another on the opposite side,

and each fashions itself into a new polype, the practical effect will be

that before long you will see a single polype converted into a sort of

tree or bush of polypes. And these will all remain associated together,

like a kind of co-operative store, which is a thing I believe you

understand very well here,--each mouth will help to feed the body and

each part of the body help to support the multifarious mouths. I think

that is as good an example of a zoological co-operative store as you can

well have. Such are these wonderful creatures. But they are capable not

only of multiplying in this way, but in other ways, by having a more

ordinary and regular kind of offspring. Little eggs are hatched and the

young are passed out by the way of the mouth, and they go swimming

about as little oval bodies covered with a very curious kind of hairlike

processes. Each of these processes is capable of striking water like an

oar; and the consequence is that the young creature is propelled through

the water. So that you have the young polype floating about in this

fashion, covered by its 'vibratile cilia', as these long filaments,

which are capable of vibration are termed. And thus, although the polype

itself may be a fixed creature unable to move about, it is able to

spread its offspring over great areas. For these creatures not only

propel themselves, but while swimming about in the sea for many hours,

or perhaps days, it will be obvious that they must be carried hither and

thither by the currents of the sea, which not unfrequently move at the

rate of one or two miles an hour. Thus, in the course of a few days,

the offspring of this stationary creature may be carried to a very great

distance from its parent; and having been so carried it loses these

organs by which it is propelled, and settles down upon the bottom of the

sea and grows up again into the form and condition of its parents. So

that if you suppose a single polype of this kind settled upon the bottom

of the sea, it may by these various methods--that is to say, by cutting

itself in two, which we call "fission," or by budding; or by sending out

these swimming embryos,--multiply itself to an enormous extent, and

give rise to thousands, or millions, of progeny in a comparatively short

time; and these thousands, or millions, of progeny may cover a very

large surface of the sea bottom; in fact, you will readily perceive

that, give them time, and there is no limit to the surface which they

may cover.

Having understood thus far the general nature of these polypes, which

are the fabricators both of the red and white coral, let us consider a

little more particularly how the skeletons of the red coral and of

the white coral are formed. The red coral polype perches upon the sea

bottom, it then grows up into a sort of stem, and out of that stem there

grow branches, each of which has its own polypes; and thus you have a

kind of tree formed, every branch of the tree terminated by its polype.

It is a tree, but at the end of the branches there are open mouths of

polypes instead of flowers. Thus there is a common soft body connecting

the whole, and as it grows up the soft body deposits in its interior a

quantity of carbonate of lime, which acquires a beautiful red or flesh

colour, and forms a kind of stem running through the whole, and it is

that stem which is the red coral. The red coral grows principally at

the bottom of the Mediterranean Sea, at very great depths, and the coral

fishers, who are very adventurous seamen, take their drag nets, of a

peculiar kind, roughly made, but efficient for their purpose, and drag

them along the bottom of the sea to catch the branches of the red coral,

which become entangled and are thus brought up to the surface. They are

then allowed to putrefy, in order to get rid of the animal matter, and

the red coral is the skeleton that is left.

In the case of the white coral, the skeleton is more complete. In the

red coral, the skeleton belongs to the whole; in the white coral there

is a special skeleton for every one of these polypes in addition to that

for the whole body. There is a skeleton formed in the body of each of

them, like a cup divided by a number of radiating partitions towards the

outside; and that cup is formed of carbonate of lime, only not stained

red, as in the case of the red coral. And all these cups are joined

together into a common branch, the result of which is the formation of

a beautiful coral tree. This is a great mass of madrepore, and in the

living state every one of the ends of these branches was terminated by

a beautiful little polype, like a sea anemone, and all the skeleton

was covered by a soft body which united the polypes together. You must

understand that all this skeleton has been formed in the interior of the

body, to suit the branched body of the polype mass, and that it is as

much its skeleton as our own bones are our skeleton. In this next coral

the creature which has formed the skeleton has divided itself as it

grew, and consequently has formed a great expansion; but scattered

all over this surface there were polype bodies like those I previously

described. Again, when this great cup was alive, the whole surface was

covered with a beautiful body upon which were set innumerable small

polype flowers, if we may so call them, often brilliantly coloured;

and the whole cup was built up in the same fashion by the deposit of

carbonate of lime in the interior of the combined polype body, formed

by budding and by fission in the way I described. You will perceive that

there is no necessary limit to this process. There is no reason why we

should not have coral three or four times as big; and there are certain

creatures of this kind that do fabricate very large masses, or half

spheres several feet in diameter. Thus the activity of these animals

in separating carbonate of lime from the sea and building it up into

definite shapes is very considerable indeed.

Now I think I have said sufficient--as much as I can without taking you

into technical details, of the general nature of these creatures which

form coral. The animals which form coral are scattered over the seas of

all countries in the world. The red coral is comparatively limited, but

the polypes which form the white coral are widely scattered. There

are some of them which remain single, or which give rise to only small

accumulations; and the skeletons of these, as they die, accumulate upon

the bottom of the sea, but they do not come to much; they are washed

about and do not adhere together, but become mixed up with the mud of

the sea. But there are certain parts of the world in which the coral

polypes which live and grow are of a kind which remain, adhere together,

and form great masses. They differ from the ordinary polypes just in

the same way as those plants which form a peat-bog or meadow-turf differ

from ordinary plants. They have a habit of growing together in masses

in the same place; they are what we call "gregarious" things; and the

consequence of this is, that as they die and leave their skeletons,

those skeletons form a considerable solid aggregation at the bottom

of the sea, and other polypes perch upon them, and begin building upon

them, and so by degrees a great mass is formed. And just as we know

there are some ancient cities in which you have a British city, and over

that the foundations of a Roman city; and over that a Saxon city, and

over that again a modern city, so in these localities of which I am

speaking, you have the accumulations of the foundations of the houses,

if I may use the term, of nation after nation of these coral polypes;

and these accumulations may cover a very considerable space, and may

rise in the course of time from the bottom to the surface of the sea.

Mariners have a name which they apply to all sorts of obstacles

consisting of hard and rocky matter which comes in their way in the

course of their navigation; they call such obstacles "reefs," and they

have long been in the habit of calling the particular kind of reef,

which is formed by the accumulation of the skeletons of dead corals, by

the name of "coral reefs," therefore, those parts of the world in which

these accumulations occur have been termed by them "coral reef areas,"

or regions in which coral reefs are found. There is a very notable

example of a simple coral reef about the island of Mauritius, which I

dare say you all know, lies in the middle of the Indian Ocean. It is a

very considerable and beautiful island, and is surrounded on all sides

by a mass of coral, which has been formed in the way I have described;

so that if you could get upon the top of one of the peaks of the island,

and look down upon the Indian Ocean, you would see that the beach round

the Island was continued outward by a kind of shallow terrace, which

is covered by the sea, and where the sea is quite shallow; and at a

distance varying from three-quarters of a mile to a mile and a half from

the proper beach, you would see a line of foam or surf which looks most

beautiful in contrast with the bright green water in the inside, and the

deep blue of the sea beyond. That line of surf indicates the point at

which the waters of the ocean are breaking upon the coral reef which

surrounds the island. You see it sweep round the island upon all sides,

except where a river may chance to come down, and that always makes a

gap in the shore.

There are two or three points which I wish to bring clearly before your

notice about such a reef as this. In the first place, you perceive

it forms a kind of fringe round the island, and is therefore called a

"fringing reef." In the next place, if you go out in a boat, and take

soundings at the edge of the reef, you find that the depth of the water

is not more than from 20 to 25 fathoms--that is about 120 to 150 feet.

Outside that point you come to the natural sea bottom; but all inside

that depth is coral, built up from the bottom by the accumulation of the

skeletons of innumerable generations of coral polypes. So that you see

the coral forms a very considerable rampart round the island. What the

exact circumference may be I do not remember, but it cannot be less than

100 miles, and the outward height of this wall of coral rock nowhere

amounts to less than about 100 or 150 feet.

When the outward face of the reef is examined, you find that the upper

edge, which is exposed to the wash of the sea, and all the seaward face,

is covered with those living plant-like flowers which I have described

to you. They are the coral polypes which grow, flourish, and add to the

mass of calcareous matter which already forms the reef. But towards the

lower part of the reef, at a depth of about 120 feet, these creatures

are less active, and fewer of them at work; and at greater depths than

that you find no living coral polype at all; and it may be laid down

as a rule, derived from very extensive observation, that these

reef-building corals cannot live in a greater depth of water than about

120 to 150 feet. I beg you to recollect that fact, because it is one I

shall have to come back to by and by, and to show to what very curious

consequences that rule leads. Well then, coming back to the margin of

the reef, you find that part of it which lies just within the surf to be

coated by a very curious plant, a sort of seaweed, which contains in its

substance a very great deal of carbonate of lime, and looks almost like

rock; this is what is called the nullipore. More towards the land,

we come to the shallow water upon the inside of the reef, which has

a particular name, derived from the Spanish or the Portuguese--it is

called a "lagoon," or lake. In this lagoon there is comparatively little

living coral; the bottom of it is formed of coral mud. If we pounded

this coral in water, it would be converted into calcareous mud, and the

waves during storms do for the coral skeletons exactly what we might do

for this coral in a mortar; the waves tear off great fragments and

crush them with prodigious force, until they are ground into the merest

powder, and that powder is washed into the interior of the lagoon, and

forms a muddy coating at the bottom. Beside that there are a great many

animals that prey upon the coral--fishes, worms, and creatures of that

kind, and all these, by their digestive processes, reduce the coral to

the same state, and contribute a very important element to this fine

mud. The living coral found in the lagoon, is not the reef building

coral; it does not give rise to the same massive skeletons. As you go

in a boat over these shallow pools, you see these beautiful things,

coloured red, blue, green, and all colours, building their houses;

but these are mere tenements, and not to be compared in magnitude

and importance to the masses which are built by the reef-builders

themselves. Now such a structure as this is what is termed a "fringing

reef." You meet with fringing reefs of this kind not only in the

Mauritius, but in a number of other parts of the world. If these were

the only reefs to be seen anywhere, the problem of the formation of

coral reefs would never have been a difficult one. Nothing can be

easier than to understand how there must have been a time when the coral

polypes came and settled on the shores of this island, everywhere within

the 20 to 25 fathom line, and how, having perched there, they gradually

grew until they built up the reef.

But these are by no means the only sort of coral reefs in the world; on

the contrary, there are very large areas, not only of the Indian ocean,

but of the Pacific, in which many many thousands of square miles

are covered either with a peculiar kind of reef, which is called the

"encircling reef," or by a still more curious reef which goes by the

name of the "atoll." There is a very good picture, which Professor

Roscoe has been kind enough to prepare for me, of one of these atolls,

which will enable you to form a notion of it as a landscape. You have in

the foreground the waters of the Pacific. You must fancy yourself in the

middle of the great ocean, and you will perceive that there is an almost

circular island, with a low beach, which is formed entirely of coral

sand; growing upon that beach you have vegetation, which takes, of

course, the shape of the circular land; and then, in the interior of the

circle, there is a pool of water, which is not very deep--probably in

this case not more than eight or nine fathoms--and which forms a strange

and beautiful contrast to the deep blue water outside. This circular

island, or atoll, with a lagoon in the middle, is not a complete circle;

upon one side of it there is a break, exactly like the entrance into a

dock; and, as a matter of course, these circular islets, or atolls, form

most efficient break-waters, for if you can only get inside your ship is

in perfect safety, with admirable anchorage in the interior. If the ship

were lying within a mile of that beach, the water would be one or

two thousand feet deep; therefore, a section of that atoll, with the

soundings as deep as this all round, would give you the notion of a

great cone, cut off at the top, and with a shallow cup in the middle of

it. Now, what a very singular fact this is, that we should have rising

from the bottom of the deep ocean a great pyramid, beside which all

human pyramids sink into the most utter insignificance! These singular

coral limestone structures are very beautiful, especially when crowned

with cocoa-nut trees. There you see the long line of land, covered with

vegetation--cocoa-nut trees--and you have the sea upon the inner and

outer sides, with a vessel very comfortably riding at anchor. That is

one of the remarkable forms of reef in the Pacific. Another is a sort of

half-way house, between the atoll and the fringing reef; it is what is

called an "encircling reef." In this case you see an Island rising out

of the sea, and at two or three miles distance, or more, and separated

by a deep channel, which may be eight to twelve fathoms deep, there is a

reef, which encircles it like a great girdle; and outside that again the

water is one or two thousand feet deep. I spent three or four years

of my life in cruising about a modification of one of these encircling

reefs, called a "barrier reef," upon the east coast of Australia--one of

the most wonderful accumulations of coral rock in the world. It is about

1,100 miles long, and varies in width from one or two to many miles.

It is separated from the coast of Australia by a channel of about 25

fathoms deep; while outside, looking toward America, the water is two or

three thousand feet deep at a mile from the edge of the reef. This is an

accumulation of limestone rock, built up by corals, to which we have no

parallel anywhere else. Imagine to yourself a heap of this material more

than one thousand miles long, and several miles wide. That is a

barrier reef; but a barrier reef is merely as it were a fragment of an

encircling reef running parallel to the coast of a great continent.

I told you that the polypes which built these reefs were not able to

live at a greater depth than 20 to 25 fathoms of water; and that is the

reason why the fringing reef goes no farther from the land than it does.

And for the same reason, if the Pacific could be laid bare we should

have a most singular spectacle. There would be a number of mountains

with truncated tops scattered over it, and those mountains would have an

appearance just the very reverse of that presented by the mountains

we see on shore. You know that the mountains on shore are covered with

vegetation at their bases, while their tops are barren or covered with

snow; but these mountains would be perfectly bare at their bases, and

all round their tops they would be covered with a beautiful vegetation

of coral polypes. And not only would this be the case, but we should

find that for a considerable distance down, all the material of these

atoll and encircling reefs was built up of precisely the same coral rock

as the fringing reef. That is to say, you have an enormous mass of coral

rock at a depth below the surface of the water where we know perfectly

well that the coral animals could not have lived to form it. When

those two facts were first put together, naturalists were quite as much

puzzled as I daresay you are, at present, to understand how these

two seeming contradictions could be reconciled; and all sorts of odd

hypotheses were resorted to. It was supposed that the coral did not

extend so far down, but that there was a great chain of submarine

mountains stretching through the Pacific, and that the coral had grown

upon them. But only fancy what supposition that was, for you would have

to imagine that there was a chain of mountains a thousand miles or more

long, and that the top of every mountain came within 20 fathoms of the

surface of the sea, and neither rose above nor sunk beneath that level.

That is highly improbable: such a chain of mountains was never known.

Then how can you possibly account for the curious circular form of the

atolls by any supposition of this kind? I believe there was some one who

imagined that all these mountains were volcanoes, and that the reefs

had grown round the tops of the craters, so we all stuck fast. I may say

"we," though it was rather before my time. And when we all stick fast,

it is just the use of a man of genius that he comes and shows us the

meaning of the thing. He generally gives an explanation which is so

ridiculously simple that everybody is ashamed that he did not find

it out before; and the way such a discoverer is often rewarded is by

finding out that some one had made the discovery before him! I do not

mean to say that it was so in this particular instance, because the

great man who played the part of Columbus and the egg on this occasion

had, I believe, always had the full credit which he so well deserves.

The discoverer of the key to these problems was a man whose name you

know very well in connection with other matters, and I should not wonder

if some of you have heard it said that he was a superficial kind of

person who did not know much about the subject on which he writes. He

was Mr. Darwin, and this brilliant discovery of his was made public

thirty years ago, long before he became the celebrated man he now

is; and it was one of the most singular instances of that astonishing

sagacity which he possesses of drawing consequences by way of deduction

from simple principles of natural science--a power which has served him

in good stead on other occasions. Well, Mr. Darwin, looking at these

curious difficulties and having that sort of knowledge of natural

phenomena in general, without which he could not have made a step

towards the solution of the problem, said to himself--"It is perfectly

clear that the coral which forms the base of the atolls and fringing

reefs could not possibly have been formed there if the level of the sea

has always been exactly where it is now, for we know for certain that

these polypes cannot build at a greater depth than 20 to 25 fathoms, and

here we find them at 50 to 100 fathoms."

That was the first point to make clear. The second point to deal with

was--if the polypes cannot have built there while the level of the

sea has remained stationary, then one of two things must have

happened--either the sea has gone up, or the land has gone down.

There is no escape from one of these two alternatives. Now the

objections to the notion of the sea having gone up are very considerable

indeed; for you will readily perceive that the sea could not possibly

have risen a thousand feet in the Pacific without rising pretty much

the same distance everywhere else; and if it had risen that height

everywhere else since the reefs began to be formed, the geography of

the world in general must have been very different indeed, at that time,

from what it is now. And we have very good means of knowing that any

such rise as this certainly has not taken place in the level of the sea

since the time that the corals have been building their houses. And so

the only other alternative was to suppose that the land had gone down,

and at so slow a rate that the corals were able to grow upward as fast

as it went downward. You will see at once that this is the solution of

the mystery, and nothing can be simpler or more obvious when you come to

think about it. Suppose we start with a coral sea and put in the middle

of it an island such as the Mauritius. Now let the coral polypes come

and perch on the shore and build a fringing reef, which will stop when

they come to 20 or 25 fathoms, and you will have a fringing reef like

that round the island in the illustration. So long as the land remains

stationary, so long as it does not descend so long will that reef be

unable to get any further out, because the moment the polype embryos

try to get below they die. But now suppose that the land sinks very

gradually indeed. Let it subside by slow degrees, until the mountain

peak, which we have in the middle of it, alone projects beyond the sea

level. The fringing reef would be carried down also; but we suppose that

the sinking is so slow that the coral polypes are able to grow up as

fast as the land is carried down; consequently they will add layer upon

layer until they form a deep cup, because the inner part of the reef

grows much more slowly than the outer part. Thus you have the reef

forming a bed thicker upon the flanks of the island; but the edge of the

reef will be very much further out from the land, and the lagoon will be

many times deeper; in short, your fringing reef will be converted into

an encircling reef. And if, instead of this being an island, it were a

great continent like Australia, then you will have the phenomenon of a

barrier reef which I have described. The barrier reef of Australia

was originally a fringing reef; the land has gone slowly down; the

consequence is the lagoon has deepened until its depth is now 25 fathoms

and the corals have grown up at the outer edge until you have that

prodigious accumulation which forms the barrier reef at present. Now let

this process go on further still; let us take the land a further step

down, so as to submerge even the peak. The coral, still growing up, will

cover the surface of the land, and you will have an atoll reef; that is

to say, a more or less circular or oval ring of coral rock with a lagoon

in the middle. Thus you see that every peculiarity and phenomenon

of these different forms of coral reef was explained at once by the

simplest of all possible suppositions, namely, by supposing that the

land has gone down at a rate not greater than that at which the coral

polypes have grown up. You explain a Fringing Reef as a reef which is

formed round land comparatively stationary; an Encircling Reef as one

which is formed round land going down; and an Atoll as a reef formed

upon land gone down; and the thing is so simple that a child may

understand it when it is once explained.

But this would by no means satisfy the conditions of a scientific

hypothesis. No man who is cautious would dream of trusting to an

explanation of this kind simply because it explained one particular set

of facts. Before you can possibly be safe in dealing with Nature--who is

very properly made of the feminine gender, on account of the astonishing

tricks which she plays upon her admirers!--I say before you can be safe

in dealing with Nature, you must get two or three kinds of cross proofs,

so as to make sure not only that your hypothesis fits that particular

set of facts, but that it is not contradicted by some other set of facts

which is just as clear and certain. And it so happens, that in this case

Mr. Darwin supplied the cross proofs as well as the immediate evidence.

You have all heard of volcanoes, those wonderful vents in the surface of

the earth out of which pour masses of lava, cinders and ashes, and

the like. Now, it is a matter of observation and experience that all

volcanoes are placed in areas in which the surface of the earth is

undergoing elevation, or at any rate is stationary; they are not placed

in parts of the world in which the level of the land is being lowered.

They are all indications of a great subterranean activity, of a

something being pushed up, and therefore naturally the land either gives

way and lets it come through, or else is raised up by its violence. And

so Mr. Darwin, being desirous not to merely put out a flashy hypothesis,

but to get at the truth of the matter, said to himself, "If my notion of

this matter is right, then atolls and encircling reefs, inasmuch as they

are dependent upon subsidence, ought not to be found in company with

volcanoes; and, 'vice versa', volcanoes ought not to be found in company

with atolls, but they ought to be found in company with fringing reefs."

And if you turn to Mr. Darwin's great work upon the coral reefs, you

will see a very beautiful chart of the world, which he prepared with

great pains and labour, showing the distribution on the one hand of the

reefs, and on the other of the volcanoes; you will find that in no case

does the atoll accompany the volcano, or the volcano burst up among the

atolls. It is most instructive to look at the great area of the Pacific

on the map, and see the great masses of atolls forming in one region of

it a most enormous belt, running from north-west to south-east; while

the volcanoes, which are very numerous in that region, go round the

margin, so that we can picture the Pacific to ourselves a section of a

kind of very shallow basin--shallow in proportion to its width, with the

atolls rising from the bottom of it, and at the margins the volcanoes.

It is exactly as if you had taken a flat mass and lifted up the edges

of it; the subterranean force which lifted up the edges shows itself

in volcanoes, and as the edges have been raised, the middle part of

the mass has gone down. In other words, the facts of physical geography

precisely and exactly correspond with the hypothesis which accounts for

the infinite varieties of coral reefs.

One other point, before I conclude, about this matter. These reefs, as

you have just perceived, are in a most singular and unexpected manner

indications of physical changes of elevations and depressions going on

upon the surface of the globe. I dare say it may have surprised you to

hear me talk in this familiar sort of way of land going up and down;

but it is one of the universal lessons of geology that the land is

going down and going up, and has been going up and down, in all sorts

of places and to all sorts of distances, through all recorded time.

Geologists would be quite right in maintaining the seeming paradox that

the stable thing in the world is the fluid sea and the shifting thing is

the solid land. That may sound a very hard saying at first, but the more

you look into geology, the more you will see ground for believing that

it is not a mere paradox.

In an unexpected manner, again, these reefs afford us not only an

indication of change of place, but they afford an indication of lapse of

time. The reef is a timekeeper of a very curious character; and you can

easily understand why. The coral polype, like everything else, takes a

certain time to grow to its full size; it does not do it in a minute;

just as a child takes a certain time to grow into a man so does the

embryo polype take time to grow into a perfect polype and form

its skeleton. Consequently every particle of coral limestone is an

expression of time. It must have taken a certain time to separate the

lime from the sea water. It is not possible to arrive at an accurate

computation of the time it must have taken to form these coral islands,

because we lack the necessary data; but we can form a rough calculation,

which leads to very curious and striking results. The computations of

the rate at which corals grow are so exceedingly variable, that we must

allow the widest possible margin for error; and it is better in this

case to make the allowance upon the side of excess. I think that anybody

who knows anything about the matter will tell you that I am making a

computation far in excess of what is probable, if I say that an inch of

coral limestone may be added to one of these reefs in the course of

a year. I think most naturalists would be inclined to laugh at me for

making such an assumption, and would put the growth at certainly not

more than half that amount. But supposing it is so, what a very curious

notion of the antiquity of some of these great living pyramids comes out

by a very simple calculation. There is no doubt whatever that the sea

faces of some of them are fully a thousand feet high, and if you take

the reckoning of an inch a year, that will give you 12,000 years for the

age of that particular pyramid or cone of coral limestone; 12,000 long

years have these creatures been labouring in conditions which must have

been substantially the same as they are now, otherwise the polypes could

not have continued their work. But I believe I very much understate both

the height of some of these masses, and overstate the amount which these

animals can form in the course of a year; so that you might very safely

double the period as the time during which the Pacific Ocean, the

general state of the climate, and the sea, and the temperature has been

substantially what it is now; and yet that state of things which now

obtains in the Pacific Ocean is the yesterday of the history of the life

of the globe. Those pyramids of coral rock are built upon a foundation

which is itself formed by the deposits which the geologist has to deal

with. If we go back in time and search through the series of the rocks,

we find at every age of the world's history which has yet been examined,

accumulations of limestone, many of which have certainly been built

up in just the same way as those coral reefs which are now forming the

bottom of the Pacific Ocean. And even if we turn to the oldest periods

of geologic history, although the nature of the materials is changed,

although we cannot apply to them the same reasonings that we can to the

existing corals, yet still there are vast masses of limestone formed of

nothing else than the accumulations of the skeletons of similar animals,

and testifying that even in those remote periods of the world's history,

as now, the order of things implies that the earth had already endured

for a period of which our ordinary standards of chronology give us not

the slightest conception. In other words, the history of these coral

reefs, traced out honestly and carefully, and with the same sort of

reasoning that you would use in the ordinary affairs of life, testifies,

like every fact that I know of, to the prodigious antiquity of the earth

since it existed in a condition in the main similar to that in which it

now is.

End of Coral and Coral Reefs.