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 above, as every rock between London
and Birmingham or Exeter has been laid down. That I only assert now.
But I do not wish you to take it on trust from me. I wish to prove
it to you as I go on, or to do what is far better for you: to put
you in the way of proving it for yourself, by using your common
sense.
At the risk of seeming prolix, I must say a few more words on this
matter. I have special reasons for it. Until I can get you to "let
your thoughts play freely" round this question of the superposition
of soils and rocks, there will be no use in my going on with these
papers.
Suppose then (to argue from the known to the unknown) that you were
watching men cleaning out a pond.
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