The whole surface of the earth,--I speak broadly, and leave out minor
qualifications,--is made up of such layers of mud, so hard, the
majority of them, that we call them rock whether limestone or
sandstone, or other varieties of rock. And, seeing that every part of
the crust of the earth is made up in this way, you might think that the
determination of the chronology, the fixing of the time which it has
taken to form this crust is a comparatively simple matter. Take a
broad average, ascertain how fast the mud is deposited upon the bottom
of the sea, or in the estuary of rivers; take it to be an inch, or two,
or three inches a year, or whatever you may roughly estimate it at;
then take the total thickness of the whole series of stratified rocks,
which geologists estimate at twelve or thirteen miles, or about seventy
thousand feet, make a sum in short division, divide the total thickness
by that of the quantity deposited in one year, and the result will, of
course, give you the number of years which the crust has taken to form.

Truly, that looks a very simple process! It would be so except for
certain difficulties, the very first of which is that of finding how
rapidly sediments are deposited; but the main difficulty--a difficulty
which renders any certain calculations of such a matter out of the
question--is this, the sea-bottom on which the deposit takes place is
continually shifting.

Instead of the surface of the earth being that stable, fixed thing that
it is popularly believed to be, being, in common parlance, the very
emblem of fixity itself, it is incessantly moving, and is, in fact, as
unstable as the surface of the sea, except that its undulations are
infinitely slower and enormously higher and deeper.