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.

(FIGURE 4. Section through deposits on sea-bottom and shore.)

Let C y (Figure 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 is going on. Now, suppose
that the whole land, C, D, which we have regarded as stationary, goes