C.D. Warner, et al., comp. The Library of the World’s Best Literature.
An Anthology in Thirty Volumes. 1917.
On a Piece of Chalk
By Thomas Henry Huxley (1825–1895)
A
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 towards 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 the 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 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 Globigerinæ and granules. 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….
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 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 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 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 1,700 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 1,700 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, though 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, though 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, 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 whole population 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 bowlder 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 bowlder clay forming a vast mass, which lies upon the chalk, and must consequently have come into existence after it. Huge bowlders 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 bowlder 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 bowlder 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 boles 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 the spoils of which 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 bowlder 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….
A small beginning has led us to a great ending. If I were to put the bit of chalk with which we started into the hot but obscure flame of burning hydrogen, it would presently shine like the sun. It seems to me that this physical metamorphosis is no false image of what has been the result of our subjecting it to a jet of fervent though nowise brilliant thought to-night. It has become luminous; and its clear rays, penetrating the abyss of the remote past, have brought within our ken some stages of the evolution of the earth. And in the shifting, “without haste but without rest,” of the land and sea, as in the endless variation of the forms assumed by living beings, we have observed nothing but the natural product of the forces originally possessed by the substance of the universe.