When you hammer a piece of iron with a hammer, you will soon find it get quite warm.

When you squeeze the air together in your pop-gun, you actually make the air inside warmer, till the pellet flies out, and the air expands and cools again.

Nay, I believe you cannot hold up a stone on the palm of your hand without that stone after a while warming your hand, because it presses against you in trying to fall, and you press against it in trying to hold it up.

And recollect above all the great and beautiful example of that law which you were lucky enough to see on the night of the 14th of November 1867, how those falling stars, as I told you then, were coming out of boundless space, colder than any ice on earth, and yet, simply by pressing against the air above our heads, they had their motion turned into heat, till they burned themselves up into trains of fiery dust.

So remember that wherever you have pressure you have heat, and that the pressure of the upper rocks upon the lower is quite enough, some think, to account for the older and lower rocks being harder than the upper and newer ones.
But why should the lower rocks be older and the upper rocks newer?
You told me just now that the high mountains in Wales were ages older than Windsor Forest, upon which we stand: but yet how much lower we are here than if we were on a Welsh mountain.
Ah, my dear child, of course that puzzles you, and I am afraid it must puzzle you still till we have another talk; or rather it seems to me that the best way to explain that puzzle to you would be for you and me to go a journey into the far west, and look into the matter for ourselves; and from here to the far west we will go, either in fancy or on a real railroad and steamboat, before we have another talk about these things.
Now it is time to stop.