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 is going on. Now,
suppose that the whole land, C, D, which we have regarded as stationary,
goes down, as it does so, both A and B go further out from the shore,
which will be at yl; x1, y1, being the new sea-level. The consequence
will be that the layer of mud (A), being now, for the most part,
further than the force of the current is strong enough to convey even
the finest 'debris', will, of course, receive no more deposits, and
having attained a certain thickness will now grow no thicker.
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