trata; and I shall follow his plan.

I must ask you meanwhile to remember one law or rule, which seems to
me founded on common sense; namely, that the uppermost strata are
really almost always the newest; that when two or more layers,
whether of rock or earth--or indeed two stones in the street, or two
sheets on a bed, or two books on a table--any two or more lifeless
things, in fact, lie one on the other, then the lower one was most
probably put there first, and the upper one laid down on the lower.
Does that seem to you a truism? Do I seem almost impertinent in
asking you to remember it? So much the better. I shall be saved
unnecessary trouble hereafter.

But some one may say, and will have a right to say, "Stop--the lower
thing may have been thrust under the upper one." Quite true: and
therefore I said only that the lower one was most probably put there
first. And I said "most probably," because it is most probable that
in nature we should find things done by the method which costs least
force, just as you do them. I will warrant that when you want to
hide a thing, you lay something down on it ten times for once that
you thrust it under something else. You may say, "What? When I want
to hide a paper, say, under the sofa-cover, do I not thrust it
under?"

No, you lift up the cover, and slip the paper in, and let the cover
fall on it again. And so, even in that case, the paper has got into
its place first.

Now why is this? Simply because in laying one thing on another you
only move weight. In thrusting one thing under another, you have not
only to move weight, but to overcome friction. That is why you do
it, though you are hardly aware of it: simply because so you employ
less force, and take less trouble.

And so do clays and sands and stones. They are laid down on each
other, and not thrust under each other, because thus less force is
expended in getting them into place.

There are exceptions. There are cases in which nature does try to
thrust one rock under another. But to do that she requires a force
so enormous, compared with what is employed in laying one rock on
another, that (so to speak) she continually fails; and instead of
producing a volcanic eruption, produces only an earthquake. Of that
I may speak hereafter, and may tell you, in good time, how to
distinguish rocks which have been thrust in from beneath, from rocks
which have been laid down from