almost entirely made up of _Globigerinæ_, 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 silicious bodies belong partly
to the lowly vegetable organisms which are called _Diatomaceæ_, 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 silicious 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.
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 _Globigerinæ_ 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 _Globigerinæ_ 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 _Globigerinæ_, in
proportion to other organisms of like kind, increases with the depth of
the sea; and that deep-water _Globigerinæ_ are larger than those which
live in 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.[30]
However, the important points for us are, that the living _Globigerinæ_
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 _Globigerinæ_ 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 _Globigerinæ_, 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. _Globigerinæ_, 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.[31]
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
_Globigerinæ_; 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 _Globigerinæ_ 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 _Globigerinæ_, 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.
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.
No less certain is it that the time during which the countries we now
call south-east 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 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 _Globigerinæ_ 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.
There is more curious evidence, again, that the process of covering up,
or, in other words, the deposit of _Globigerinæ_ skeletons, did not go
on very fast. It is demonstrable that an animal of the cretaceous sea
might die, that its skeleton might lie uncovered upon the sea-bottom
long enough to lose all its outward coverings and appendages by
putrefaction; and that, after this had happened, another animal might
attach itself to the dead and naked skeleton, might grow to maturity,
and might itself die before the calcareous mud had buried the whole.
Cases of this kind are admirably described by Sir Charles Lyell. He
speaks of the frequency with which geologists find in the chalk a
fossilized sea-urchin, to which is attached the lower valve of a
_Crania_. This is a kind of shell-fish, with a shell composed of two
pieces, of which, as in the oyster, one is fixed and the other free.
"The upper valve is almost invariably wanting, though occasionally found
in a perfect state of preservation in the white chalk at some distance.
In this case, we see clearly that the sea-urchin first lived from youth
to age, then died and lost its spines, which were carried away. Then
the young _Crania_ adhered to the bared shell, grew and perished in its
turn; after which, the upper valve was separated from the lower, before
the Echinus became enveloped in chalky mud."[32]
A specimen in the Museum of Practical Geology, in London, still further
prolongs the period which must have elapsed between the death of the
sea-urchin, and its burial by the _Globigerinæ_. For the outward face of
the valve of a _Crania_, which is attached to a sea-urchin
(_Micraster_), is itself overrun by an incrusting coralline, which
spreads thence over more or less of the surface of the sea-urchin. It
follows that, after the upper valve of the _Crania_ fell off, the
surface of the attached valve must have remained exposed long enough to
allow of the growth of the whole coralline, since corallines do not live
imbedded in mud.
The progress of knowledge may, one day, enable us to deduce from such
facts as these the maximum rate at which the chalk can have accumulated,
and thus to arrive at the minimum duration of the chalk period. Suppose
that the valve of the _Crania_ upon which a coralline has fixed itself
in the way just described, is so attached to the sea-urchin that no part
of it is more than an inch above the face upon which the sea-urchin
rests. Then, as the coralline could not have fixed itself, if the
_Crania_ had been covered up with chalk mud, and could not have lived
had itself been so covered, it follows, that an inch of chalk mud could
not have accumulated within the time between the death and decay of the
soft parts of the sea-urchin and the growth of the coralline to the full
size which it has attained. If the decay of the soft parts of the
sea-urchin; the attachment, growth to maturity, and decay of the
_Crania_; and the subsequent attachment and growth of the coralline,
took a year (which is a low estimate enough), the accumulation of the
inch of chalk must have taken more than a year: and the deposit of a
thousand feet of chalk must, consequently, have taken more than twelve
thousand years.
The foundation of all this calculation is, of course, a knowledge of the
length of time the _Crania_ and the coralline needed to attain their
full size; and, on this head, precise knowledge is at present wanting.
But there are circumstances which tend to show, that nothing like an
inch of chalk has accumulated during the life of a _Crania_; and, on any
probable estimate of the length of that life, the chalk period must have
had a much longer duration than that thus roughly assigned to it.
Thus, not only is it certain that the chalk is the mud of an ancient
sea-bottom; but it is no less certain, that the chalk sea existed during
an extremely long period, though we may not be prepared to give a
precise estimate of the length of that period in years. The relative
duration is clear, though the absolute duration may not be definable.
The attempt to affix any precise date to the period at which the chalk
sea began, or ended, its existence, is baffled by difficulties of the
same kind. But the relative age of the cretaceous epoch may be
determined with as great ease and certainty as the long duration of that
epoch.
You will have heard of the interesting discoveries recently made, in
various parts of Western Europe, of flint implements, obviously worked
into shape by human hands, under circumstances which show conclusively
that man is a very ancient denizen of these regions.
It has been proved that the old populations of Europe, whose existence
has been revealed to us in this way, consisted of savages, such as the
Esquimaux are now; that, in the country which is now France, they hunted
the reindeer, and were familiar with the ways of the mammoth and the
bison. The physical geography of France was in those days different from
what it is now--the river Somme, for instance, having cut its bed a
hundred feet deeper between that time and this; and, it is probable,
that the climate was more like that of Canada or Siberia, than that of
Western Europe.
The existence of these people is forgotten even in the traditions of the
oldest historical nations. The name and fame of them had utterly
vanished until a few years back; and the amount of physical change which
has been effected since their day, renders it more than probable that,
venerable as are some of the historical nations, the workers of the
chipped flints of Hoxne or of Amiens are to them, as they are to us, in
point of antiquity.
But, if we assign to these hoar relics of long vanished generations of
men the greatest age that can possibly be claimed for them, they are not
older than the drift, or boulder clay, which, in comparison with the
chalk, is but a very juvenile deposit. You need go no further than your
own sea-board for evidence of this fact. At one of the most charming
spots on the coast of Norfolk, Cromer, you will see the boulder clay
forming a vast mass, which lies upon the chalk, and must consequently
have come into existence after it. Huge boulders of chalk are, in fact,
included in the clay, and have evidently been brought to the position
they now occupy, by the same agency as that which has planted blocks of
syenite from Norway side by side with them.
The chalk, then, is certainly older than the boulder clay. If you ask
how much, I will again take you no further than the same spot upon your
own coasts for evidence. I have spoken of the boulder clay and drift as
resting upon the chalk. That is not strictly true. Interposed between
the chalk and the drift is a comparatively insignificant layer,
containing vegetable matter. But that layer tells a wonderful history.
It is full of stumps of trees standing as they grew. Fir-trees are there
with their cones, and hazel-bushes with their nuts; there stand the
stools of oak and yew trees, beeches and alders. Hence this stratum is
appropriately called the "forest-bed."
It is obvious that the chalk must have been upheaved and converted into
dry land, before the timber trees could grow upon it. As the bolls of
some of these trees are from two to three feet in diameter, it is no
less clear that the dry land thus formed remained in the same condition
for long ages. And not only do the remains of stately oaks and
well-grown firs testify to the duration of this condition of things, but
additional evidence to the same effect is afforded by the abundant
remains of elephants, rhinoceroses, hippopotamuses, and other great wild
beasts, which it has yielded to the zealous search of such men as the
Rev. Mr. Gunn.
When you look at such a collection as he has formed, and bethink you
that these elephantine bones did veritably carry their owners about,
and these great grinders crunch, in the dark woods of which the
forest-bed is now the only trace, it is impossible not to feel that they
are as good evidence of the lapse of time as the annual rings of the
tree-stumps.
Thus there is a writing upon the wall of cliffs at Cromer, and whoso
runs may read it. It tells us, with an authority which cannot be
impeached, that the ancient sea-bed of the chalk sea was raised up, and
remained dry land, until it was covered with forest, stocked with the
great game whose spoils have rejoiced your geologists. How long it
remained in that condition cannot be said; but "the whirligig of time
brought its revenges" in those days as in these. That dry land, with the
bones and teeth of generations of long-lived elephants, hidden away
among the gnarled roots and dry leaves of its ancient trees, sank
gradually to the bottom of the icy sea, which covered it with huge
masses of drift and boulder clay. Sea-beasts, such as the walrus, now
restricted to the extreme north, paddled about where birds had twittered
among the topmost twigs of the fir-trees. How long this state of things
endured we know not, but at length it came to an end. The upheaved
glacial mud hardened into the soil of modern Norfolk. Forests grew once
more, the wolf and the beaver replaced the reindeer and the elephant;
and at length what we call the history of England dawned.
Thus you have, within the limits of your own county, proof that the
chalk can justly claim a very much greater antiquity than even the
oldest physical traces of mankind. But we may go further and
demonstrate, by evidence of the same authority as that which testifies
to the existence of the father of men, that the chalk is vastly older
than Adam himself.
The Book of Genesis informs us that Adam, immediately upon his creation,
and before the appearance of Eve, was placed in the Garden of Eden. The
problem of the geographical position of Eden has greatly vexed the
spirits of the learned in such matters, but there is one point
respecting which, so far as I know, no commentator has ever raised a
doubt. This is, that of the four rivers which are said to run out of it,
Euphrates and Hiddekel are identical with the rivers now known by the
names of Euphrates and Tigris.
But the whole country in which these mighty rivers take their origin,
and through which they run, is composed of rocks which are either of the
same age as the chalk, or of later date. So that the chalk must not only
have been formed, but, after its formation, the time required for the
deposit of these later rocks, and for their upheaval into dry land, must
have elapsed, before the smallest brook which feeds the swift stream of
"the great river, the river of Babylon," began to flow.
Thus, evidence which cannot be rebutted, and which need not be
strengthened, though if time permitted I might indefinitely increase its
quantity, compels you to believe that the earth, from the time of the
chalk to the present day, has been the theatre of a series of changes as
vast in their amount, as they were slow in their progress. The area on
which we stand has been first sea and then land, for at least four
alternations; and has remained in each of these conditions for a period
of great length.
Nor have these wonderful metamorphoses of sea into land, and of land
into sea, been confined to one corner of England. During the chalk
period, or "cretaceous epoch," not one of the present great physical
features of the globe was in existence. Our great mountain ranges,
Pyrenees, Alps, Himalayas, Andes, have all been upheaved since the chalk
was deposited, and the cretaceous sea flowed over the sites of Sinai and
Ararat.
All this is certain, because rocks of cretaceous, or still later, date
have shared in the elevatory movements which gave rise to these mountain
chains; and may be found perched up, in some cases, many thousand feet
high upon their flanks. And evidence of equal cogency demonstrates that,
though, in Norfolk, the forest-bed rests directly upon the chalk, yet it
does so, not because the period at which the forest grew immediately
followed that at which the chalk was formed, but because an immense
lapse of time, represented elsewhere by thousands of feet of rock, is
not indicated at Cromer.
I must ask you to believe that there is no less conclusive proof that a
still more prolonged succession of similar changes occurred, before the
chalk was deposited. Nor have we any reason to think that the first term
in the series of these changes is known. The oldest sea-beds preserved
to us are sands, and mud, and pebbles, the wear and tear of rocks which
were formed in still older oceans.
But, great as is the magnitude of these physical changes of the world,
they have been accompanied by a no less striking series of modifications
in its living inhabitants.
All the great classes of animals, beasts of the field, fowls of the
air, creeping things, and things which dwell in the waters, flourished
upon the globe long ages before the chalk was deposited. Very few,
however, if any, of these ancient forms of animal life were identical
with those which now live. Certainly not one of the higher animals was
of the same species as any of those now in existence. The beasts of the
field, in the days before the chalk, were not our beasts of the field,
nor the fowls of the air such as those which the eye of men has seen
flying, unless his antiquity dates infinitely further back than we at
present surmise. If we could be carried back into those times, we should
be as one suddenly set down in Australia before it was colonized. We
should see mammals, birds, reptiles, fishes, insects, snails, and the
like, clearly recognisable as such, and yet not one of them would be
just the same as those with which we are familiar, and many would be
extremely different.
From that time to the present, the population of the world has undergone
slow and gradual, but incessant, changes. There has been no grand
catastrophe--no destroyer has swept away the forms of life of one
period, and replaced them by a totally new creation; but one species has
vanished and another has taken its place; creatures of one type of
structure have diminished, those of another have increased, as time has
passed on. And thus, while the differences between the living creatures
of the time before the chalk and those of the present day appear
startling, if placed side by side, we are led from one to the other by
the most gradual progress, if we follow the course of Nature through
the whole series of those relics of her operations which she has left
behind.
And it is by the population of the chalk sea that the ancient and the
modern inhabitants of the world are most completely connected. The
groups which are dying out flourish, side by side, with the groups which
are now the dominant forms of life.
Thus the chalk contains remains of those strange flying and swimming
reptiles, the pterodactyl, the ichthyosaurus, and the plesiosaurus,
which are found in no later deposits, but abounded in preceding ages.
The chambered shells called ammonites and belemnites, which are so
characteristic of the period preceding the cretaceous, in like manner
die with it.
But, amongst these fading remainders of a previous state of things, are
some very modern forms of life, looking like Yankee pedlars among a
tribe of Red Indians. Crocodiles of modern type appear; bony fishes,
many of them very similar to existing species, almost supplant the forms
of fish which predominate in more ancient seas; and many kinds of living
shell-fish first become known to us in the chalk. The vegetation
acquires a modern aspect. A few living animals are not even
distinguishable as species, from those which existed at that remote
epoch. The _Globigerina_ of the present day, for example, is not
different specifically from that of the chalk; and the same may be said
of many other _Foraminifera_. I think it probable that critical and
unprejudiced examination will show that more than one species of much
higher animals have had a similar longevity; but the only example which
I can at present give confidently is the snake's-head lamp-shell
(_Terebratulina caput serpentis_), which lives in our English seas and
abounded (as _Terebratulina striata_ of authors) in the chalk.