Kitabı oku: «Scientific Essays and Lectures», sayfa 6

THOUGHTS IN A GRAVEL-PIT ⁶

Ladies and gentlemen, we may of course think of anything which we choose in a gravel-pit, as we may anywhere else.  Thought is free: at least so we fancy.

But the most right sort of thought, after all, is thought about what lies nearest us; not always, but surely once in a way, that we may understand something of everyday objects.  And therefore it may be well worth our while to go once into a gravel-pit, and think about it, till we have learnt what a gravel-pit is.

Learnt what a gravel-pit is?  Everybody knows.

If it be so, everybody knows more than I know.  We all know a gravel-pit when we see one; but we do not all know what we see.  I do not know.  I know a little; a few scraps of fact about these pits round here, though about no others.  Were I to go into a pit a hundred miles, even fifty miles off, I could tell you nothing certain about it; perhaps might make a dozen mistakes.  But what I know, with tolerable certainty, about the pits round here, I wish to tell you to-night.

But why?  You do not need, one in ten of you, to know anything about gravel, unless you be highway surveyor, or have a garden-walk to make; and then someone will easily tell you where the best gravel is to be got, at so much a load.

Very true; but you come here to-night to instruct yourselves; that is, to learn, if you can, something more about the world you live in; something more about God who made the world.

And you come here to educate yourselves; to educe and bring out your own powers of perceiving, judging, reasoning; to improve yourselves in the art of all arts, which is, the art of learning.  That is mental education.

Now if a gravel-pit will teach you a little about these things, you will surely call it a rich gravel-pit.  If it helps you to wisdom, which is worth more than gold; which is the only way to get gold wisely, and spend it wisely; then we will call our pit no more a gravel-pit, but a wisdom-pit, a mine of wisdom.

Let us go out, then, in fancy (for it is too cold to go out in person) to Hook Common, scramble down into the first gravel-pit we come to, and see what we can see.

The first thing we see is a quantity of stones, more or less rounded, lying in gravel and poor clay.

Well—what do those stones tell us?

These stones, as I told you when I addressed you last, are ancient and venerable worthies.  They have seen a great deal in their time.  They have had a great deal of knocking about, and have stood it manfully.  They have stood the knocking about of three worlds already; and have done their duty therein; and they are ready (if you choose to mend the road with them) to stand the knocking about of this fourth world, and being most excellent gravel, to do their duty in this world likewise; which is more, I fear, than either you or I can say for ourselves.

Three worlds?

Yes.  Standing there in the gravel-pit, I see three old worlds, in each of which these stones played their part; and this world of man for the fourth, and the best of all—for man if not for the stones.  I speak sober truth.  Let me explain it step by step.

You know the chalk-hills to the south; and the sands of Crooksbury and the Hind Head beyond them.  There is one world.

You know the clays and sands of Hook and Newnham, Dogmersfield and Shapley Heath, and all the country to the north as far as Reading.  There is a second world.

You know the gravel-pit itself; and all the upper soils and gravels, which are spread over the length and breadth of the country to the north.  There is a third world.

Let us take them one by one.

First, the chalk.

The chalk-hills rise much higher than the surrounding country; but you must not therefore suppose that they were made after it, and laid on the top of it.  That guess would be true, if you went south-east from here toward the Hind Head.  The chalk lies on the top of the sands of Crooksbury Hill, and the clays of Holt Forest; but it dips underneath the sands of Shapley Heath, and the clays of Dogmersfield, and reappears from underneath them again at Reading.

Thus you at Odiham stand on the edge of a chalk basin; of what was once a sea, or estuary, with shores of chalk, which begins at the foot of the High Clere Hills, and runs eastward, widening as it goes, past London, into the Eastern Sea.  Everywhere under this great basin is the floor of chalk, covered with clays and sands, which, for certain reasons, are called by geologists Tertiary strata.

But what has this to do with a gravel-pit?

This first.  That all the flints in this pit have come out of the chalk.  They are coloured, most of them, with iron, which has turned them brown; but they are exactly the same flints as those gray ones in the chalk-pit on the other side of the town.

How do I know that?

I think our own eyes will prove it: they are the same shapes, and of the same substance; but as a still surer proof, we find exactly the same fossils in them; sponges, choanites (which were something like our modern sea-anemones), corals, and “shepherds’ crowns” as the boys call the fossil sea-urchins.  The species of all these, and of other fossils, in the chalk-pit and in the gravel-pit, are absolutely identical.  The natural conclusion is, then, that the gravel has been formed from the washings of the chalk.  The white lime of the chalk has been carried away in water by some flood or floods; the heavier flints have been left behind.

Stop now one moment, and think.  You all know how very few flints there are in the chalk-pit, in proportion to the mass of chalk.  You all know what vast gravel-beds cover the country to the north, and often to the thickness of many feet.  Try and conceive, then, what a much more vast mass of chalk must have been washed away, to leave that vast mass of gravel behind it.—Conceive?  It is past conception.  I will but give you two hints as to its probable size.

The chalk to the eastward, between here and Farnham, is a far narrower and shallower band than anywhere else in England.  Its narrowest point is, I believe, beneath the bishop’s palace at Farnham, where it may be a hundred feet thick, instead of several hundred, as it usually is in other parts of England.  The cause of this is, that the whole of the upper chalk has been washed away, to form the gravel-beds to the north and east of us.

Again.  Some of you may have been on the Hind Head or on Leith Hill, and have looked southward over the glorious prospect of the rich Weald, spread out five hundred feet below—a sight to make an Englishman proud of his native land.  Now, the mass of chalk which has been carried away began behind you, at the Hogsback, and the line of chalk-hills which runs to Boxhill, and stretched hundreds of feet above your head as you stand on Hind Head or Leith Hill, right over the old Weald of Sussex to the chalk of the South Downs.  And out of the scourings of that vast mass of chalk was our gravel-pit made.

Of that, and also of the Hind Head sands below it.

For you will find a great deal of sharp sand in our gravel-pits, which has not, I believe, come from the grinding of chalk flints; for if it had been ground, it would not be the sharp sand it is; the particles would be rounded off at the edges.  This is probably sand from the Hind Head; from what geologists term the greensands, below the chalk.

And I have a better proof of this—at least I should have in every gravel-pit at Eversley—in a few pieces of a stone which is not chalk-flint at all; flattish and oblong, not more than two or three inches in diameter; of a grayish colour, and a porous worm-eaten surface, which no chalk-flint ever has.  They are chert, which abound in the greensand formation; and insignificant as they look, are a great token of a most important fact; that the currents which formed our sands and gravels set from the south during a long series of ages, first till they had washed away all the chalk off the Weald, and next till they had washed away a great part of the sands, which then became exposed, the remains whereof form great commons over a wide tract of Surrey.

Now let me pause, and ask you to observe one thing.  How, in inductive science, we arrive, by patient and simple observation of the things around us, at the most grand and surprising results.  Of course I am not giving you the whole of the facts which have made this argument certain.  I am only giving you enough to make it probable to you.  Its certainty has been proved by many different men, labouring in many different parts of England, and of the Continent also, and then comparing their discoveries together; often, of course, making mistakes; but each working on patiently, and correcting their early mistakes by fresh facts, till they have at last got hold of the true key to the mystery, and are as certain of the existence of the great island of the Weald, and its gradual destruction by the waves and currents of an ancient sea, as if they had seen it with their bodily eyes.  You must take all this, of course, as truth from me to-night; but you may go and examine for yourselves; and see how far your own common sense and observations agree with those of learned geologists.

The history of this great Wealden island to the south-east of us is obscure enough; but a few general facts, which bear upon our gravel-pit, I can give you.

I must begin, however, ages before the Wealden island existed; when the chalk of which its mass was composed was at the bottom of a deep ocean.

We know now what chalk is, and how it was made.  We know that it was deposited as white lime mud, at a vast sea-depth, seemingly undisturbed by winds or currents.  We know that not only the flint, but the chalk itself, is made up of shells; the shell of little microscopic animalcules smaller than a needle’s point, in millions of millions, some whole, some broken, some in powder, which lived, and died, and decayed for ages in the great chalk sea.

We know this, I say.  We had suspected it long ago, and become more and more certain of it as the years went on.  But now we seem to have a proof of it which is past gainsaying.

In the late survey of the bottom of the Atlantic Ocean, with a view to laying down the electric telegraph between England and America, by Lieutenant Maury of the American navy, a great discovery was made.  It was found that the floor of the Atlantic Ocean, after you have left the land a few hundred miles, is one vast plain of mud, of some thirteen hundred miles in breadth.  But here is the wonder; it was found that at a depth, averaging 1,600 fathoms—9,600 feet—in utter darkness, the sea floor is covered with countless millions of animalcule-shells, of the same families, though not of the same species, as those which compose the chalk.

At the bottom of a still ocean, then, the chalk was deposited.  But it took many an age to raise it to where Odiham chalk-pit now stands.

But how was it raised?

By the upheaving force of earthquakes.  Or rather, by the upheaving force which causes earthquakes, when it acts in a single shock, cracking the earth’s crust by an explosion; but which acts, too, slowly and quietly, uplifting day by day, and year by year, some portions of the earth’s surface, and letting others sink down; as in the case of the valley of the Jordan and the Dead Sea, which is now 1,300 feet below the level of the Mediterranean.

That these upheaving forces were much more violent than now, in the earlier epochs of our planet, we have some reason to believe: but the subject is too long a one to enter on now; and all I can say is, that you must conceive for yourself the chalk gradually brought up to the surface, worn away along a shifting shoreline by the waves of the sea, and covered in shallow water by the clays and sands on which Odiham stands; and which compose the earliest part of our second world.

A second world; a new world.  We can use no weaker expression.  When we compare the chalk with the strata which lie upon it, we can only call them a complete new creation.

For not only were they deposited in shallow water; a great deal of them, probably, near river-mouths, and by the force of violent currents, as the irregularity of their lower bed proves: but there is hardly a plant or animal found in the chalk itself, which is found in the gravels, sands, or clays above it.  The shells are all new species; unseen before in this planet.  The vegetables, as far as we know them, are all different from anything found in the chalk, or in the beds below it.  God Almighty, for His own good pleasure, has made all things new.  It is a very awful fact; but it is a very certain one.  Several times, in the history of our planet, has the Lord God fulfilled the words of the Psalmist:

“Thou takest away their breath, they die, and return again to their dust.

“Thou sendest forth thy breath, they are made: and thou renewest the face of the earth.”

But in no instance, perhaps, is the gulf so vast; is the leap from one world to another so sheer, as that between the chalk and the London clay above it.

But how do I know that there was a shore-line here?  And how do I know that the chalk was covered with sand-beds?

I know that there was a shore-line here, from this fact.  If you will look at the surface of the chalk, where the sands and clays lie on it, you will find that it is not smooth; that the beds do not rest conformably on each other, as if they had been laid down quietly by successive tides, while the chalk below was still soft mud.  So far from it, the chalk must have become hard rock, and have been exposed to the action of the sea waves, for centuries, perhaps, before the sands began to cover it.  For you find the surface of the chalk furrowed, worn into deep pits, which are often filled with sand, and gravel, and rounded lumps of chalk.  You may see this for yourselves, in the topmost layer of any chalk-pit round here.  You may see, even, in some places, the holes which boring shells, such as work now close to the tide-level, have made in it; all the signs, in fact, of the chalk having been a rocky sea-beach for ages.

The first bed which you will generally find upon the water-worn surface of the chalk is a layer of green-sand and green-coated flints.  Among these are met with in many places beds of a great oyster, now unknown in life.  I cannot say whether there are any here; but at Reading, to the east of Farnham, at Croydon, and under London, they are abundant.  There must have been miles and miles of oyster-bed at the bottom of that Eocene sea; among the oyster-beds, beds of a peculiar pebble, which we shall see in our gravel-pit.

They are flints; but very small, dark, often almost black, and quite round and polished.  Compare them with the average flints of the pit, and you see that while the average flints are fresh from the chalk, these have plainly been rolled and rounded for years.  They are (except in their dark colour) exactly such shingle as forms the south-coast beach about Hastings and Brighton.  They are the shingle beaches of the Eocene sea, part of which are preserved under the London clay.  To the north a vast bed of them remains in its original place, on Blackheath near London; while part, in the district to the south, which the London clay has not covered, have been washed away, and carried into our gravel-pit, to mingle with other flints fresh from the chalk.

I said just now that I had proof that a great tract of the chalk-hills which are now bare, was once covered with sand and gravel.  Here, in the presence of these dark pebbles, is a proof.  But I have another, and a yet more curious one.

For our gravel-pit, if it be, will possibly yield us another, and a more curious object.  You most of you have seen, I dare say, large stones, several feet long, taken out of these pits.  In the gravels and sands at Pirbright they are so plentiful that they are quarried for building-stone.  And good building-stone they make; being exceedingly hard, so that no weather will wear them away.  They are what is called saccharine (that is, sugary) sandstone.  If you chip off a bit, you find it exactly like fine whity-brown sugar, only intensely hard.  Now these stones have become very famous; for two reasons.  First, the old Druids used them to build their temples.  Second, it is a most puzzling question where they came from.

First.  They were used to build Druid temples.

If you go to the further lodge of Dogmersfield Park, which opens close to the Barley-mow Inn, you will see there several of them, about five feet high each, set up on end.  They run in a line through the plantation past the lodge, along the park palings; one or two are in an adjoining field.  They are the remains of a double line; an avenue of stones, which has formed part of an ancient British temple.

I know no more than that: of that I am certain.

But if you go to the Chalk Downs of Wiltshire, you see these temples in their true grandeur.  You have all heard of Stonehenge on Salisbury Plain.  Some of you may have heard of the great Druid temple at Abury in Wilts, which, were it not all but destroyed, would be even grander than Stonehenge.  These are made of this same sugar-sandstone.

But where did the sandstone come from?  You may say, it “grew” of itself in our sands and gravels; but it certainly did not “grow” on the top of a bare chalk down.  The Druids must have brought the stones thither, then, from neighbouring gravel-pits.  They brought them, no doubt: but not from gravel-pits.  The stones are found loose on the downs on the top of the bare chalk, in places where they plainly have not been put by man.

For instance, near Marlborough is a long valley in the chalk, which, for perhaps half a mile, is full of huge blocks of this sandstone, lying about on the turf.  The “gray wethers” the shepherds call them.  One look at them would show you that no man’s hand had put them there.  They look like a river of stone, if I may so speak; as if some mighty flood had rolled them along down the valley, and there left them behind as it sunk.

Now, whence did they come?

Many answers have been given to that question.  It was supposed by many learned men that they had been brought from the sandstone mountains of Wales, like the rolled pebbles of which I spoke just now.  But the answer to that was, that these great stones are not rolled: they are all squarish, more or less; their edges are often sharp and fresh, instead of being polished almost into balls, as they would have been in rolling two hundred miles along a sea-bottom, before such a tremendous current as would have been needed to carry them.

Then rose a very clever guess.  They must have been carried by icebergs, as much silt and stones (we know) has been carried, and have dropped, like them, to the bottom, when the icebergs melted.

There is great reason in that; but we have cause now to be certain that they did not come from Wales.  That they are not pieces of a rock older than the chalk, but much younger; that they were very probably formed close to where they now lie.

Now—how do we know that?

If you are not tired with all this close reasoning, I will tell you.—If you are, say so: but as I said at first, I want to show you what steady and sharp head-work this same geology requires, even in the nearest gravel-pit.

Well, then.  I do not think our gravel-pit will tell us what we want: but I know one which will.

You have all heard of Lady Grenville’s lovely place, Dropmore, beyond Maidenhead; where the taste of that good and great man, the late Lord Grenville, converted into a paradise of landscape-gardening art a barren common, full of clay and gravel-pits.  Lord Grenville wanted stones for rockwork; in those pits he found some blocks, of the same substance as those of Stonehenge or Pirbright.  And they contain the answer.  The upper surface of most of them is the usual clear sugar-sandstone: but the under surface of many has round pebbles imbedded in it, looking just like plums in a pudding; the smaller above and the larger below, as if they had sunk slowly through the fluid sand, before the whole mass froze, as it were, suddenly together.  And these pebbles are nothing else than rolled chalk flints.

That settles the matter.  The pebbles could not come from Wales; there are no flints there.  They could not have been made before the chalk; for out of the chalk they came; and the only explanation which is left to us, I believe, is, that over the tops of the chalk downs; over our heads where we stand now, there once stretched layers of sand and gravel, “Tertiary strata” as I have been calling them to you; and among them layers of this same hard sandstone.

When the floods came they must have swept away all these soft sands and gravels (possibly to make the Bagshot sands, of which I shall speak presently), and left the chalk downs bare; but while they had strength to move the finer particles, they had not generally strength to move these sandstone blocks, but let them drop through, and remain upon the freshly-bared floor of chalk, as the only relics of a tertiary land long since swept away; while some were carried off, possibly by icebergs, as far as Pirbright, and dropped, as the icebergs melted, both there, at Dogmersfield, and also, though few and small, in Eversley and the neighbourhood.

But how came these tertiary sandstones to be so very hard, while the strata around them are so soft?

Ladies and gentlemen, I know no more than you.  Experience seems to say that stone will not harden into that sugary crystalline state, save under the influence of great heat: but I do not know how the heat should have got to that layer in particular.  Possibly there may have been eruptions of steam, of boiling water holding silex (flint) in solution—a very rare occurrence: but something similar is still going on in the famous Geysers or boiling springs of Iceland.  However, I have no proof that this was the cause.  I suppose we shall find out some day how it happened; for we must never despair of finding out anything which depends on facts.

Part of the town of Odiham, and of North Warnborough, stands, I believe, upon these lower beds, which are called by geologists the Woolwich and Reading beds, and the Plastic clays, from the good brick earth which is so often found among them.  But as soon as you get to Hook Common, and to Dogmersfield Park, you enter on a fresh deposit; the great bed of the London clay.

I give you a rough section, from a deep well at Dogmersfield House; from which you may see how steeply the chalk dips down here under the clay, so that Odiham stands, as it were, on the chalk beach of the clay sea.

In boring that well there were pierced:

Forty feet of the upper sands (the Bagshot sands), of which I shall speak presently.

Three hundred and thirty feet of London clay.

Then about forty feet of mottled clays and sands.

Whether the chalk was then reached, I do not know.  It must have been close below.  But these mottled clays and sands abound in water (being indeed the layer which supplies the great breweries in London, and those soda-water bottles on dumb-waiters which squirt in Trafalgar Square); and (I suppose) the water being reached, the boring ceased.

Now, this great bed of London clay, even more than the sands below it, deserves the title of a new creation.

As a proof—some of you may recollect, when the South-Western Railway was in making, seeing shells—some of them large and handsome ones—Nautili, taken out of the London clay cutting near Winchfield.

Nautili similar to them (but not the same) are now only found in the hottest parts of the Indian seas; and what is more, not one of those shells is the same as the shells you find in the chalk.  Throughout this great bed of London clay, the shells, the remains of plants and animals, are altogether a new creation.  If you look carefully at the London clay shells, you will be struck with their general likeness to fresh East Indian shells; and rightly so.  They do approach our modern live shells in form, far more than any which preceded them; and indeed, a few of the London clay shells exist still in foreign seas; in the beds, again, above the clay, you will meet with still more species which are yet alive; while in the chalk, and below the chalk, you never meet, I believe, with a single recent shell.  It is for this reason that the London clay is said to be Eocene, that is, the dawn of the new creation.

The chalk, I told you, seems to have been deposited at the bottom of a still and deep ocean.  But the London clay, we shall find, was deposited in a comparatively shallow sea, least in depth toward High Clere on the west, and deepening towards London and the mouth of the Thames.

For not only is the clay deeper as you travel eastward, but—and this is a matter to which geologists attach great importance—the character of the shells differs in different parts of the clay.

You must know that certain sorts of shells live in deep water, and certain in shallow.  You may prove this to yourselves, on a small scale, whenever you go to the seaside.  You will find that the shell which crawl on the rocks about high-water mark are different from those which you find at low-tide mark; and those again different from the shells which are brought up by the oyster-dredgers from the sea outside.  Now, the lower part of the clay, near here, contains shallow-water shells: but if you went forty miles to the eastward, you would find in the corresponding lower beds of the clay, deep-water shells, and far above them, shallow-water shells such as you find here: a fact which shows plainly that this end of the clay sea was shallowest, and therefore first filled up.

But again—and this is a very curious fact—between the time of the Plastic clays and sands, with their oyster-beds and black pebbles, and that of the London clay, great changes had taken place.  The Plastic clay and sands were deposited during a period of earthquake, of upheaval and subsidence of ancient lands; and therefore of violent currents and flood waves, seemingly rushing down from, or round the shores of that Wealden island to the south of us, on the shore of which island Odiham once stood.  We know this from the great irregularity of the beds: while the absence of that irregularity proves to us that the London clay was deposited in a quiet sea.

But more.  A great change in the climate of this country had taken place meanwhile; slowly perhaps: but still it had taken place.

In the lowest clay above the chalk are found at Reading many leaves, and buds, and seeds of trees, showing that there was dry land near; and these trees, as far as the best botanists can guess, were trees like those we have in England now.  Not of the same species, of course: but still trees belonging to a temperate climate, which had its regular warm summer and cold winter.

But before the London clay had been all deposited, this temperate climate had changed to a tropical one; and the plants and animals of the upper part of the London clay had begun to resemble rather those of the mouths of the African slave-rivers.

Extraordinary as this is, it is certainly true.

We know that the country near the mouth of the Thames, and probably the land round us here, was low rich soil, some half under water, some overflowed by rivers; some by fresh or brackish pools.  We know all this; for we find the shells which belong to a shallow sea, mixed with fresh-water ones.  We know, too, that the climate of this rich lowland was a tropical one.  We know that the neighbourhood of the Isle of Sheppey, at the mouth of the Thames, was covered with rich tropic vegetation; with screw pines and acacias, canes and gourds, tenanted by opossums, bats, and vultures: that huge snakes twined themselves along the ground, tortoises dived in the pools, and crocodiles basked on the muds, while the neighbouring seas swarmed with sharks as huge and terrible as those of a West Indian shore.

It is all very wonderful, ladies and gentlemen: but be it is: and all we can say is, with the Mussulman—“God is great.”

And then—when, none knows but God—there came a time in which some convulsion of nature changed the course of the sea currents, and probably destroyed a vast tract of land between England and France, and probably also, that sunken island of Atlantis of which old Plato dreamed—the vast tract which connected for ages Ireland, Cornwall, Brittany, and Portugal.  That convulsion covered up the rich clays with those barren sands and gravels, which now rise in flat and dreary steppes, on the Beacon Hill, Aldershot Moors, Hartford Bridge Flat, Frimley ridges, and Windsor Forest.  That rich old world was all swept away, and instead of it desolation and barrenness, piling up slowly on its ruins a desert of sand and shingle, rising inch by inch out of a lifeless sea.  There is something very awful to me in the barrenness of those Bagshot sands, after the rich tropic life of the London clay.  Not a fossil is to be found in them for miles.  Save a few shells, I believe, near Pirbright, there is not a hint that a living being inhabited that doleful sea.

But do not suppose, gentlemen and ladies, that we have yet got our gravel-pit made, or that the way-worn pebbles of which it is composed are near the end of their weary journey.  Poor old stones!  Driven out of their native chalk, rolled for ages on a sea-beach, they have tried to get a few centuries’ sleep in the Eocene sands on the top of the chalk hills behind us, while the London clay was being deposited peacefully in the tropic sea below; and behold, they are swept out, once more, and hurled pell-mell upon the clay, two hundred feet over our heads.

Over our heads, remember.  We have come now to a time when Hartford Bridge Flats stretched away to the Beacon Hill, and many a mile to the south-eastward—even down into Kent, and stretched also over Winchfield and Dogmersfield hither.

What broke them up?  What furrowed out their steep side-valleys?  What formed the magnificent escarpment of the Beacon Hill, or the lesser one of Finchamstead Ridges?  What swept away all but a thin cap of them on the upper part of Dogmersfield Park, another under Winchfield House; another at Bearwood, and so forth?