[24] "Philosophie Positive," i. p. 99.
[25] Ibid., i. p. 77.
[26] "Philosophie Positive," i. p. 78.
[27] Loc. cit., Preface Speciale, pp. i. ii.
[28] Mr. Congreve leaves out these important words, which show that I refer to the spirit, and not to the details of science.
IX.
ON A PIECE OF CHALK.
A LECTURE TO WORKING MEN.
If a well were to be sunk at our feet in the midst of the city of Norwich, the diggers would very soon find themselves at work in that white substance almost too soft to be called rock, with which we are all familiar as "chalk."
Not only here, but over the whole county of Norfolk, the well-sinker might carry his shaft down many hundred feet without coming to the end of the chalk; and, on the sea-coast, where the waves have pared away the face of the land which b.r.e.a.s.t.s them, the scarped faces of the high cliffs are often wholly formed of the same material. Northward, the chalk may be followed as far as Yorkshire; on the south coast it appears abruptly in the picturesque western bays of Dorset, and breaks into the Needles of the Isle of Wight; while on the sh.o.r.es of Kent it supplies that long line of white cliffs to which England owes her name of Albion.
Were the thin soil which covers it all washed away, a curved band of white chalk, here broader, and there narrower, might be followed diagonally across England from Lulworth in Dorset, to Flamborough Head in Yorkshire--a distance of over 280 miles as the crow flies.
From this band to the North Sea, on the east, and the Channel, on the south, the chalk is largely hidden by other deposits; but, except in the Weald of Kent and Suss.e.x, it enters into the very foundation of all the south-eastern counties.
Attaining, as it does in some places, a thickness of more than a thousand feet, the English chalk must be admitted to be a ma.s.s of considerable magnitude. Nevertheless, it covers but an insignificant portion of the whole area occupied by the chalk formation of the globe, which has precisely the same general characters as ours, and is found in detached patches, some less, and others more extensive, than the English.
Chalk occurs in north-west Ireland; it stretches over a large part of France,--the chalk which underlies Paris being, in fact, a continuation of that of the London basin; it runs through Denmark and Central Europe, and extends southward to North Africa; while, eastward, it appears in the Crimea and in Syria, and may be traced as far as the sh.o.r.es of the Sea of Aral, in Central Asia.
If all the points at which true chalk occurs were circ.u.mscribed, they would lie within an irregular oval about 3,000 miles in long diameter--the area of which would be as great as that of Europe, and would many times exceed that of the largest existing inland sea--the Mediterranean.
Thus the chalk is no unimportant element in the masonry of the earth"s crust, and it impresses a peculiar stamp, varying with the conditions to which it is exposed, on the scenery of the districts in which it occurs. The undulating downs and rounded coombs, covered with sweet-gra.s.sed turf, of our inland chalk country, have a peacefully domestic and mutton-suggesting prettiness, but can hardly be called either grand or beautiful. But, on our southern coasts, the wall-sided cliffs, many hundred feet high, with vast needles and pinnacles standing out in the sea, sharp and solitary enough to serve as perches for the wary cormorant, confer a wonderful beauty and grandeur upon the chalk headlands. And, in the East, chalk has its share in the formation of some of the most venerable of mountain ranges, such as the Lebanon.
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 pa.s.sages in the history of man can be supported by such an overwhelming ma.s.s 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 a.s.sert, 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 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 stalact.i.tes, 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 una.s.sisted 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 ma.s.s 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 _Globigerinae_ 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.
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 water may, under certain conditions, a.s.sume 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 a.s.sume the form and structure we find in fossils. If any one were to try to persuade you that an oyster-sh.e.l.l (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-sh.e.l.ls 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 a.n.a.logy 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 mult.i.tudes, literally more numerous than the sands of the sea-sh.o.r.e, 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 circ.u.mstance 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 sh.e.l.ls, 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) Lieut. 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, Lieut. 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 _Globigerinae_ already known to occur in the chalk.
Thus far, the work had been carried on simply in the interests of science, but Lieut. 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 a.s.signed 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.[29]
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 down hill 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 sh.o.r.e.
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 greyish-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, greyish 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 a.s.sociated 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_ sh.e.l.l, 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 _Globigerinae_ 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.
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 sh.e.l.ls; 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 sh.e.l.ls and skeletons composed of silex, or pure flint. These silicious 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 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 pa.s.s 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 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.