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. 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, tho 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.
[Footnote 60: From a lecture delivered to the workingmen of Norwich,
England, during the meeting of the British Association in 1868, now
included in "Lay Sermons, Addresses and Reviews." By permission of D.
Appleton & Co.]
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, tho
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...
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 granules.
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 proportion. 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 Globigerinæ and granules. Let us fix
our attention upon the Globigerina. It is the spore 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.
The history of the discovery of these living Globigerinæ, 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 the shoals and rocks; and the more
the burden 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 the 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 10,000
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
Globigerinæ already known to occur in the 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 results 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 I,700 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 200 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
downhill for about 200 miles, to the point at which the bottom is now
covered by I,700 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, tho the depth of water
upon it now varies from 10,000 to 15,000 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 300 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 blackboard 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 Globigerinæ imbedded 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 Globigerinæ of the chalk--it is unnecessary to speak of them.
Globigerinæ 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 Globigerinæ 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 Globigerinæ of the
Atlantic sea bottom do not live and die where they are found.
As I have mentioned, the soundings from the great Atlantic plain are
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 siliceous 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 siliceous organisms, tho they are not heavier than
the lightest dust, must have fallen in some cases through 15,000 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....
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, tho we may not be prepared to give a
precise estimate of the length of that period in years. The relative
duration is clear, tho 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 whole population of Europe whose existence has been
revealed to us in this way, consisted of savages such as the Eskimo
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 of boulder clay,
which in comparison with the chalk is but a very juvenile deposit. You
need go no further than your own seaboard 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....
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 can not 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 the spoils of which have rejoiced your geologists. How long
it remained in that condition can not 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.