ormed afterwards. Here, you see, is the first
step in the history--these layers of mud give us an idea of time.

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.

Now, what is the effect of this oscillation? Take the case to which
I have previously referred. The finer or coarser sediments that are
carried down by the current of the river, will only be carried out a
certain distance, and eventually, as we have already seen, on reaching
the stiller part of the ocean, will be deposited at the bottom.

Let C y (Fig. 4) be the sea-bottom, y D the shore, x y the sea-level,
then the coarser deposit will subside over the region B, the finer over
A, while beyond A there will be no deposit at all; and, consequently, no
record will be kept, simply because no deposit i