Young Folks' Library, Volume XI (of 20) by Various

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What is this wide-spread component of the surface of the earth? and
whence did it come?

You may think this no very hopeful inquiry. You may not unnaturally
suppose that the attempt to solve such problems as these can lead to
no result, save that of entangling the inquirer in vague speculations,
incapable of refutation and of verification.

If such were really the case, I should have selected some other
subject than a "piece of chalk" for my discourse. But, in truth,
after much deliberation, I have been unable to think of any topic
which would so well enable me to lead you to see how solid is the
foundation upon which some of the most startling conclusions of
physical science rest.

A great chapter of the history of the world is written in the chalk.
Few passages in the history of man can be supported by such an
overwhelming mass of direct and indirect evidence as that which
testifies to the truth of the fragment of the history of the globe,
which I hope to enable you to read, with your own eyes, to-night.

[Illustration: MICROSCOPIC SECTION OF CHALK.

(Magnified nearly 300 times.)

1. Textularia. 2. Globigerina. 3. Rotalia. 4. Coccoliths.]

Let me add, that few chapters of human history have a more profound
significance for ourselves. I weigh my words well when I assert, that
the man who should know the true history of the bit of chalk which
every carpenter carries about in his breeches' pocket, though ignorant
of all other history, is likely, if he will think his knowledge out to
its ultimate results, to have a truer, and therefore a better,
conception of this wonderful universe, and of man's relation to it,
than the most learned student who is deep-read in the records of
humanity and ignorant of those of nature.

The language of the chalk is not hard to learn, not nearly so hard as
Latin, if you only want to get at the broad features of the story it
has to tell; and I propose that we now set to work to spell that story
out together.

We all know that if we "burn" chalk, the result is quicklime. Chalk,
in fact, is a compound of carbonic acid gas and lime; and when you
make it very hot, the carbonic acid flies away and the lime is left.

By this method of procedure we see the lime, but we do not see the
carbonic acid. If, on the other hand, you were to powder a little
chalk and drop it into a good deal of strong vinegar, there would be a
great bubbling and fizzing, and, finally, a clear liquid, in which no
sign of chalk would appear. Here you see the carbonic acid in the
bubbles; the lime, dissolved in the vinegar, vanishes from sight.
There are a great many other ways of showing that chalk is essentially
nothing but carbonic acid and quicklime. Chemists enunciate the result
of all the experiments which prove this, by stating that chalk is
almost wholly composed of "carbonate of lime."

It is desirable for us to start from the knowledge of this fact,
though it may not seem to help us very far toward what we seek. For
carbonate of lime is a widely-spread substance, and is met with under
very various conditions. All sorts of limestones are composed of more
or less pure carbonate of lime. The crust which is often deposited by
waters which have drained through limestone rocks, in the form of what
are called stalagmites and stalactites, is carbonate of lime. Or, to
take a more familiar example, the fur on the inside of a tea-kettle is
carbonate of lime; and, for anything chemistry tells us to the
contrary, the chalk might be a kind of gigantic fur upon the bottom of
the earth-kettle, which is kept pretty hot below.

Let us try another method of making the chalk tell us its own history.
To the unassisted eye chalk looks simply like a very loose and open
kind of stone. But it is possible to grind a slice of chalk down so
thin that you can see through it--until it is thin enough, in fact, to
be examined with any magnifying power that may be thought desirable. A
thin slice of the fur of a kettle might be made in the same way. If it
were examined microscopically, it would show itself to be a more or
less distinctly laminated mineral substance, and nothing more.

But the slice of chalk presents a totally different appearance when
placed under the microscope. The general mass of it is made up of very
minute granules; but, imbedded in this matrix, are innumerable bodies,
some smaller and some larger, but, on a rough average, not more than a
hundredth of an inch in diameter, having a well-defined shape and
structure. A cubic inch of some specimens of chalk may contain
hundreds of thousands of these bodies, compacted together with
incalculable millions of the granules.

[Illustration: CHALK.

(Magnified nearly 100 diameters.)]

The examination of a transparent slice gives a good notion of the
manner in which the components of the chalk are arranged, and of
their relative proportions. But, by rubbing up some chalk with a brush
in water and then pouring off the milky fluid, so as to obtain
sediments of different degrees of fineness, the granules and the
minute rounded bodies may be pretty well separated from one another,
and submitted to microscopic examination, either as opaque or as
transparent objects. By combining the views obtained in these various
methods, each of the rounded bodies may be proved to be a
beautifully-constructed calcareous fabric, made up of a number of
chambers, communicating freely with one another. The chambered bodies
are of various forms. One of the commonest is something like a
badly-grown raspberry, being formed of a number of nearly globular
chambers of different sizes congregated together. It is called
Globigerina, and some specimens of chalk consist of little else than
Globigerinae and granules.

[Illustration: GLOBIGERINA.]

Let us fix our attention upon the Globigerina. It is the spoor of the
game we are tracking. If we can learn what it is and what are the
conditions of its existence, we shall see our way to the origin and
past history of the chalk.

A suggestion which may naturally enough present itself is, that these
curious bodies are the result of some process of aggregation which has
taken place in the carbonate of lime; that, just as in winter, the
rime on our windows simulates the most delicate and elegantly
arborescent foliage--proving that the mere mineral matter may, under
certain conditions, assume the outward form of organic bodies--so this
mineral substance, carbonate of lime, hidden away in the bowels of
the earth, has taken the shape of these chambered bodies. I am not
raising a merely fanciful and unreal objection. Very learned men, in
former days, have even entertained the notion that all the formed
things found in rocks are of this nature; and if no such conception is
at present held to be admissible, it is because long and varied
experience has now shown that mineral matter never does assume the
form and structure we find in fossils. If anyone were to try to
persuade you that an oyster-shell (which is also chiefly composed of
carbonate of lime) had crystallized out of sea-water, I suppose you
would laugh at the absurdity. Your laughter would be justified by the
fact that all experience tends to show that oyster-shells are formed
by the agency of oysters, and in no other way. And if there were no
better reasons, we should be justified, on like grounds, in believing
that Globigerina is not the product of anything but vital activity.

Happily, however, better evidence in proof of the organic nature of
the Globigerinae than that of analogy is forthcoming. It so happens
that calcareous skeletons, exactly similar to the Globigerinae of the
chalk, are being formed, at the present moment, by minute living
creatures, which flourish in multitudes, literally more numerous than
the sands of the sea-shore, over a large extent of that part of the
earth's surface which is covered by the ocean.

The history of the discovery of these living Globigerinae, and of the
part which they play in rock-building, is singular enough. It is a
discovery which, like others of no less scientific importance, has
arisen, incidentally, out of work devoted to very different and
exceedingly practical interests.

When men first took to the sea, they speedily learned to look out for
shoals and rocks; and the more the burthen of their ships increased,
the more imperatively necessary it became for sailors to ascertain
with precision the depth of the waters they traversed. Out of this
necessity grew the use of the lead and sounding-line; and, ultimately,
marine-surveying, which is the recording of the form of coasts and of
the depth of the sea, as ascertained by the sounding-lead, upon
charts.

At the same time, it became desirable to ascertain and to indicate the
nature of the sea-bottom, since this circumstance greatly affects its
goodness as holding ground for anchors. Some ingenious tar, whose name
deserves a better fate than the oblivion into which it has fallen,
attained this object by "arming" the bottom of the lead with a lump of
grease, to which more or less of the sand or mud, or broken shells, as
the case might be, adhered, and was brought to the surface. But,
however well adapted such an apparatus might be for rough nautical
purposes, scientific accuracy could not be expected from the armed
lead, and to remedy its defects (especially when applied to sounding
in great depths) Lieutenant Brooke, of the American Navy, some years
ago invented a most ingenious machine, by which a considerable portion
of the superficial layer of the sea-bottom can be scooped out and
brought up, from any depth to which the lead descends.

In 1853, Lieutenant Brooke obtained mud from the bottom of the North
Atlantic, between Newfoundland and the Azores, at a depth of more than
ten thousand feet, or two miles, by the help of this sounding
apparatus. The specimens were sent for examination to Ehrenberg of
Berlin, and to Bailey of West Point, and those able microscopists
found that this deep-sea mud was almost entirely composed of the
skeletons of living organisms--the greater proportion of these being
just like the Globigerinae already known to occur in chalk.

Thus far, the work had been carried on simply in the interests of
science, but Lieutenant Brooke's method of sounding acquired a high
commercial value, when the enterprise of laying down the
telegraph-cable between this country and the United States was
undertaken. For it became a matter of immense importance to know, not
only the depth of the sea over the whole line, along which the cable
was to be laid, but the exact nature of the bottom, so as to guard
against chances of cutting or fraying the strands of that costly rope.
The Admiralty consequently ordered Captain Dayman, an old friend and
shipmate of mine, to ascertain the depth over the whole line of the
cable, and to bring back specimens of the bottom. In former days, such
a command as this might have sounded very much like one of the
impossible things which the young prince in the Fairy Tales is ordered
to do before he can obtain the hand of the princess. However, in the
months of June and July, 1857, my friend performed the task assigned
to him with great expedition and precision, without, so far as I know,
having met with any reward of that kind. The specimens of Atlantic mud
which he procured were sent to me to be examined and reported upon.

The result of all these operations is, that we know the contours and
the nature of the surface-soil covered by the North Atlantic, for a
distance of seventeen hundred miles from east to west, as well as we
know that of any part of the dry land.

It is a prodigious plain--one of the widest and most even plains in
the world. If the sea were drained off, you might drive a wagon all
the way from Valentia, on the west coast of Ireland, to Trinity Bay in
Newfoundland. And, except upon one sharp incline about two hundred
miles from Valentia, I am not quite sure that it would even be
necessary to put the skid on, so gentle are the ascents and descents
upon that long route. From Valentia the road would lie down-hill for
about two hundred miles to the point at which the bottom is now
covered by seventeen hundred fathoms of sea-water. Then would come the
central plain, more than a thousand miles wide, the inequalities of
the surface of which would be hardly perceptible, though the depth of
water upon it now varies from ten thousand to fifteen thousand feet;
and there are places in which Mont Blanc might be sunk without showing
its peak above water. Beyond this, the ascent on the American side
commences, and gradually leads, for about three hundred miles, to the
Newfoundland shore.

Almost the whole of the bottom of this central plain (which extends
for many hundred miles in a north and south direction) is covered by a
fine mud, which, when brought to the surface, dries into a grayish
white friable substance. You can write with this on a black-board, if
you are so inclined; and, to the eye, it is quite like very soft,
grayish chalk. Examined chemically, it proves to be composed almost
wholly of carbonate of lime; and if you make a section of it, in the
same way as that of the piece of chalk was made, and view it with the
microscope, it presents innumerable Globigerinae embedded in a granular
matrix.

Thus this deep-sea mud is substantially chalk. I say substantially,
because there are a good many minor differences; but as these have no
bearing on the question immediately before us--which is the nature of
the Globigerinae of the chalk--it is unnecessary to speak of them.

Globigerinae of every size, from the smallest to the largest, are
associated together in the Atlantic mud, and the chambers of many are
filled by a soft animal matter. This soft substance is, in fact, the
remains of the creature to which the Globigerina shell, or rather
skeleton, owes its existence--and which is an animal of the simplest
imaginable description. It is, in fact, a mere particle of living
jelly, without defined parts of any kind--without a mouth, nerves,
muscles, or distinct organs, and only manifesting its vitality to
ordinary observation by thrusting out and retracting from all parts of
its surface long filamentous processes, which serve for arms and legs.
Yet this amorphous particle, devoid of everything which, in the higher
animals, we call organs, is capable of feeding, growing, and
multiplying; of separating from the ocean the small proportion of
carbonate of lime which is dissolved in sea-water; and of building up
that substance into a skeleton for itself, according to a pattern
which can be imitated by no other known agency.

The notion that animals can live and flourish in the sea, at the vast
depths from which apparently living Giobigerinae have been brought up,
does not agree very well with our usual conceptions respecting the
conditions of animal life; and it is not so absolutely impossible as
it might at first sight appear to be, that the Globigerinae of the
Atlantic sea-bottom do not live and die where they are found.

[Illustration: DIATOM OOZE DREDGED FROM A DEPTH OF 1950 FEET.

(Magnified nearly 300 diameters.)]

As I have mentioned, the soundings from the great Atlantic plain are
almost entirely made up of Globigerinae, with the granules which have
been mentioned, and some few other calcareous shells; but a small
percentage of the chalky mud--perhaps at most some five per cent of
it--is of a different nature, and consists of shells and skeletons
composed of silex, or pure flint. These siliceous bodies belong partly
to the lowly vegetable organisms which are called Diatomaceae, and
partly to the minute and extremely simple animals, termed Radiolaria.
It is quite certain that these creatures do not live at the bottom of
the ocean, but at its surface--where they may be obtained in
prodigious numbers by the use of a properly constructed net. Hence it
follows that these siliceous organisms, though they are not heavier
than the lightest dust, must have fallen, in some cases, through
fifteen thousand feet of water, before they reached their final
resting-place on the ocean floor. And, considering how large a
surface these bodies expose in proportion to their weight, it is
probable that they occupy a great length of time in making their
burial journey from the surface of the Atlantic to the bottom.

[Illustration: RADIOLARIA. (_a._ Natural size. _b._ One-third natural
size.)]

But if the Radiolaria and Diatoms are thus rained upon the bottom of
the sea, from the superficial layer of its waters in which they pass
their lives, it is obviously possible that the Globigerinae may be
similarly derived; and if they were so, it would be much more easy to
understand how they obtain their supply of food than it is at present.
Nevertheless, the positive and negative evidence all points the other
way. The skeletons of the full-grown, deep-sea Globigerinae are so
remarkably solid and heavy in proportion to their surface as to seem
little fitted for floating; and, as a matter of fact, they are not to
be found along with the Diatoms and Radiolaria, in the uppermost
stratum of the open ocean.

It has been observed, again, that the abundance of Globigerinae, in
proportion to other organisms of like kind, increases with the depth
of the sea; and that deep-water Globigerinae are larger than those
which live in the shallower parts of the sea; and such facts negative
the supposition that these organisms have been swept by currents from
the shallows into the deeps of the Atlantic.

It therefore seems to be hardly doubtful that these wonderful
creatures live and die at the depths in which they are found.[1]

[Footnote 1: During the cruise of H.M.S. Bull-dog, commanded by Sir
Leopold M'Clintock, in 1860, living star-fish were brought up,
clinging to the lowest part of the sounding-line, from a depth of 1260
fathoms, midway between Cape Farewell, in Greenland, and the Rockall
banks. Dr. Wallich ascertained that the sea-bottom at this point
consisted of the ordinary Globigerina ooze, and that the stomachs of
the star-fishes were full of Globigerinae. This discovery removes all
objections to the existence of living Globigerinae at great depths,
which are based upon the supposed difficulty of maintaining animal
life under such conditions; and it throws the burden of proof upon
those who object to the supposition that the Globigerinae live and die
where they are found.]

However, the important points for us are, that the living Globigerinae
are exclusively marine animals, the skeletons of which abound at the
bottom of deep seas; and that there is not a shadow of reason for
believing that the habits of the Globigerinae of the chalk differed
from those of the existing species. But if this be true, there is no
escaping the conclusion that the chalk itself is the dried mud of an
ancient deep sea.

In working over the soundings collected by Captain Dayman, I was
surprised to find that many of what I have called the "granules" of
that mud were not, as one might have been tempted to think at first,
the mere powder and waste of Globigerinae, but that they had a definite
form and size. I termed these bodies "_coccoliths_" and doubted their
organic nature. Dr. Wallich verified my observation, and added the
interesting discovery that, not unfrequently, bodies similar to these
"coccoliths" were aggregated together into spheroids, which he termed
"_coccospheres_." So far as we knew, these bodies, the nature of which
is extremely puzzling and problematical, were peculiar to the
Atlantic soundings.

But, a few years ago, Mr. Sorby, in making a careful examination of
the chalk by means of thin sections and otherwise, observed, as
Ehrenberg had done before him, that much of its granular basis
possesses a definite form. Comparing these formed particles with those
in the Atlantic soundings, he found the two to be identical; and thus
proved that the chalk, like the soundings, contains these mysterious
coccoliths and coccospheres. Here was a further and a most interesting
confirmation, from internal evidence, of the essential identity of the
chalk with modern deep-sea mud. Globigerinae, coccoliths, and
coccospheres are found as the chief constituents of both, and testify
to the general similarity of the conditions under which both have been
formed.[2]

[Footnote 2: I have recently traced out the development of the
"coccoliths" from a diameter of 1/7000th of an inch up to their
largest size (which is about 1/1600th), and no longer doubt that they
are produced by independent organisms, which, like the Globigerinae,
live and die at the bottom of the sea.]

The evidence furnished by the hewing, facing, and superposition of the
stones of the Pyramids, that these structures were built by men, has
no greater weight than the evidence that the chalk was built by
Globigerinae; and the belief that those ancient pyramid-builders were
terrestrial and air-breathing creatures like ourselves, is not better
based than the conviction that the chalk-makers lived in the sea.

But as our belief in the building of the Pyramids by men is not only
grounded on the internal evidence afforded by these structures, but
gathers strength from multitudinous collateral proofs, and is clinched
by the total absence of any reason for a contrary belief; so the
evidence drawn from the Globigerinae that the chalk is an ancient
sea-bottom, is fortified by innumerable independent lines of evidence;
and our belief in the truth of the conclusion to which all positive
testimony tends, receives the like negative justification from the
fact that no other hypothesis has a shadow of foundation.

It may be worth while briefly to consider a few of these collateral
proofs that the chalk was deposited at the bottom of the sea.

The great mass of the chalk is composed, as we have seen, of the
skeletons of Globigerinae, and other simple organisms, imbedded in
granular matter. Here and there, however, this hardened mud of the
ancient sea reveals the remains of higher animals which have lived and
died, and left their hard parts in the mud, just as the oysters die
and leave their shells behind them, in the mud of the present seas.

[Illustration: UPPER SILURIAN CORALS AND CRUSTACEANS.]

There are, at the present day, certain groups of animals which are
never found in fresh waters, being unable to live anywhere but in the
sea. Such are the corals; those corallines which are called Polyzoa;
those creatures which fabricate the lamp-shells, and are called
Brachiopoda; the pearly Nautilus, and all animals allied to it; and
all the forms of sea-urchins and star-fishes.

Not only are all these creatures confined to salt water at the present
day, but, so far as our records of the past go, the conditions of
their existence have been the same: hence, their occurrence in any
deposit is as strong evidence as can be obtained, that that deposit
was formed in the sea. Now the remains of animals of all the kinds
which have been enumerated occur in the chalk, in greater or less
abundance; while not one of those forms of shell-fish which are
characteristic of fresh water has yet been observed in it.

When we consider that the remains of more than three thousand distinct
species of aquatic animals have been discovered among the fossils of
the chalk, that the great majority of them are of such forms as are
now met with only in the sea, and that there is no reason to believe
that any one of them inhabited fresh water--the collateral evidence
that the chalk represents an ancient sea-bottom acquires as great
force as the proof derived from the nature of the chalk itself. I
think you will now allow that I did not overstate my case when I
asserted that we have as strong grounds for believing that all the
vast area of dry land at present occupied by the chalk was once at the
bottom of the sea, as we have for any matter of history whatever;
while there is no justification for any other belief.

[Illustration: CRETACEOUS NAUTILUS.]

No less certain is it that the time during which the countries we now
call southeast England, France, Germany, Poland, Russia, Egypt,
Arabia, Syria, were more or less completely covered by a deep sea,
was of considerable duration.

We have already seen that the chalk is, in places, more than a
thousand feet thick. I think you will agree with me that it must have
taken some time for the skeletons of the animalcules of a hundredth of
an inch in diameter to heap up such a mass as that. I have said that
throughout the thickness of the chalk the remains of other animals are
scattered. These remains are often in the most exquisite state of
preservation. The valves of the shell-fishes are commonly adherent;
the long spines of some of the sea-urchins, which would be detached by
the smallest jar, often remain in their places. In a word, it is
certain that these animals have lived and died when the place which
they now occupy was the surface of as much of the chalk as had then
been deposited; and that each has been covered up by the layer of
Globigerina mud, upon which the creatures imbedded a little higher up
have, in like manner, lived and died. But some of these remains prove
the existence of reptiles of vast size in the chalk sea. These lived
their time, and had their ancestors and descendants, which assuredly
implies time, reptiles being of slow growth.

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