Dreadful indeed.  God grant that it may never happen.  More terrible changes of land and water have happened, and are happening still in the world:  but none, I think, could happen which would destroy so much civilisation and be such a loss to mankind, as that the Thames valley should become again what it was, geologically speaking, only the other day, when these gravel banks, over which we are running to Reading, were being washed out of the chalk cliffs up above at every tide, and rolled on a beach, as you have seen them rolling still at Ramsgate.

Now here we are at Reading.  There is the carriage waiting, and away we are off home; and when we get home, and have seen everybody and everything, we will look over our section once more.

But remember, that when you ran through the chalk hills to Reading, you passed from the bottom of the chalk to the top of it, on to the Thames gravels, which lie there on the chalk, and on to the London clay, which lies on the chalk also, with the Thames gravels always over it.  So that, you see, the newest layers, the London clay and the gravels, are lower in height than the limestone cliffs at Bristol, and much lower than the old mountain ranges of Devonshire and Wales, though in geological order they are far higher; and there are whole worlds of strata, rocks and clays, one on the other, between the Thames gravels and the Devonshire hills.

But how about our moors?  They are newer still, you said, than the London clay, because they lie upon it:  and yet they are much higher than we are here at Reading.

Very well said:  so they are, two or three hundred feet higher.  But our part of them was left behind, standing up in banks, while the valley of the Thames was being cut out by the sea.  Once they spread all over where we stand now, and away behind us beyond Newbury in Berkshire, and away in front of us, all over where London now stands.

How can you tell that?

Because there are little caps—­little patches—­of them left on the tops of many hills to the north of London; just remnants which the sea, and the Thames, and the rain have not eaten down.  Probably they once stretched right out to sea, sloping slowly under the waves, where the mouth of the Thames is now.  You know the sand-cliffs at Bournemouth?

Of course.

Then those are of the same age as the Bagshot sands, and lie on the London clay, and slope down off the New Forest into the sea, which eats them up, as you know, year by year and day by day.  And here were once perhaps cliffs just like them, where London Bridge now stands.

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