Part 10

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.