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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.