I would sooner answer one question of yours than tell you ten things without your asking.
Is there potash and magnesia and silicates in the soil here?
And if there is, where did they come from?
For there are no volcanos in England.
Yes.

There are such things in the soil; and little enough of them, as the farmers here know too well.

For we here, in Windsor Forest, are on the very poorest and almost the newest soil in England; and when Madam How had used up all her good materials in making the rest of the island, she carted away her dry rubbish and shot it down here for us to make the best of; and I do not think that we and our forefathers have done so very ill with it.

But where the rich part, or staple, of our soils came from first it would be very difficult to say, so often has Madam How made, and unmade, and re-made England, and sifted her materials afresh every time.
But if you go to the Lowlands of Scotland, you may soon see where the staple of the soil came from there, and that I was right in saying that there were atoms of lava in every Scotch boy's broth.

Not that there were ever (as far as I know) volcanos in Scotland or in England.