Scientific Essays and Lectures Part 6

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 b.a.l.l.s, 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 ma.s.s 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 t.i.tle of a new creation.

As a proof--some of you may recollect, when the South-Western Railway was in making, seeing sh.e.l.ls--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 sh.e.l.ls is the same as the sh.e.l.ls you find in the chalk. Throughout this great bed of London clay, the sh.e.l.ls, the remains of plants and animals, are altogether a new creation. If you look carefully at the London clay sh.e.l.ls, you will be struck with their general likeness to fresh East Indian sh.e.l.ls; and rightly so. They do approach our modern live sh.e.l.ls in form, far more than any which preceded them; and indeed, a few of the London clay sh.e.l.ls 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 sh.e.l.l. 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 sh.e.l.ls differs in different parts of the clay.

You must know that certain sorts of sh.e.l.ls 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 sh.e.l.l 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 sh.e.l.ls which are brought up by the oyster-dredgers from the sea outside. Now, the lower part of the clay, near here, contains shallow-water sh.e.l.ls: but if you went forty miles to the eastward, you would find in the corresponding lower beds of the clay, deep- water sh.e.l.ls, and far above them, shallow-water sh.e.l.ls 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 sh.o.r.es of that Wealden island to the south of us, on the sh.o.r.e 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 sh.e.l.ls 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 sh.o.r.e.

It is all very wonderful, ladies and gentlemen: but be it is: and all we can say is, with the Mussulman--"G.o.d is great."

And then--when, none knows but G.o.d--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 sh.e.l.ls, 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?

The convulsions of a third world; more fertile in animal life than those which preceded it: but also, more terrible and rapid, if possible, in its changes.

Of this third world, the one which (so to speak) immediately preceded our own, we know little yet. Its changes are so complicated that geologists have as yet hardly arranged them. But what we can see, I will sketch for you shortly.

A great continent to the south--England, probably an island at the beginning of the period, united to the continent by new beds--the Mammoth ranging up to where we now stand.

Then a period of upheaval. The German Ocean becomes dry land. The Thames, a far larger river than now, runs far eastward to join the Seine, and the Rhine, and other rivers, which altogether flow northward, in one enormous stream, toward the open sea between Scotland and Norway.

And with this, a new creation of enormous quadrupeds, as yet unknown. Countless herds of elephants pastured by the side of that mighty river, where now the Norfolk fisherman dredges their teeth and bones far out in open sea. The hippopotamus floundered in the Severn, the rhinoceros ranged over the south-western counties; enormous elk and oxen, of species now extinct, inhabited the vast fir and larch forests which stretched from Norfolk to the farthest part of Wales; hyenas and bears double the size of our modern ones, and here and there the sabre-toothed tiger, now extinct, prowled out of the caverns in the limestone hills, to seek their bulky prey.

We see, too, a period--whether the same as this, or after it, I know not yet--in which the mountains of Wales and c.u.mberland rose to the limits of eternal frost, and Snowdon was indeed Snowdon, an alp down whose valleys vast glaciers spread far and wide; while the reindeer of Lapland, the marmot of the Alps, and the musk ox of Hudson's Bay, fed upon alpine plants, a few of whose descendants still survive, as tokens of the long past age of ice. And at every successive upheaval of the western mountains the displaced waters of the ocean swept over the lower lands, filling the valley of the Thames and of the Wey with vast beds of drift gravel, containing among its chalk flints, fragments of stone from every rock between here and Wales, teeth of elephants, skulls of ox and musk ox; while icebergs, breaking away from the glaciers of the Welsh Alps, sailed down over the spot where we now are, dropping their imbedded stones and silt, to confuse more utterly than before the records of a world rocking and throbbing above the shocks of the nether fire.

At last the convulsions get weak. The German Ocean becomes sea once more; the north-western Alps sink again to a level far lower even than their present one; only to rise again, but not so high as before; sea-beaches and sea-sh.e.l.ls fill many of our lower valleys; whales by hundreds are stranded (as in the Farnham vale) where is now dry land. Gradually the sunken land begins to rise again, and falls perhaps again, and rises again after that, more and more gently each time, till as it were the panting earth, worn out with the fierce pa.s.sions of her fiery youth, has sobbed herself to sleep once more, and this new world of man is made. And among it, I know not when, or by what diluvial wave out of hundreds which swept the Pleistocene earth, was deposited our little gravel-pit, from which we started on our journey through three worlds.

When?