The Past Condition of Organic Nature | Page 5

from its mountain sources,
brawling over the stones and rocks that intersect its path, loosening,
removing, and carrying with it in its downward course the pebbles and
lighter matters from its banks, it crushes and pounds down the rocks
and earths in precisely the same way as the wearing action of the sea
waves. The matters forming the deposit are torn from the
mountain-side and whirled impetuously into the valley, more slowly
over the plain, thence into the estuary, and from the estuary they are
swept into the sea. The coarser and heavier fragments are obviously
deposited first, that is, as soon as the current begins to lose its force by
becoming amalgamated with the stiller depths of the ocean, but the
finer and lighter particles are carried further on, and eventually
deposited in a deeper and stiller portion of the ocean.

It clearly follows from this that mud gives us a chronology; for it is
evident that supposing this, which I now sketch, to be the sea bottom,
and supposing this to be a coast-line; from the washing action of the
sea upon the rock, wearing and grinding it down into a sediment of
mud, the mud will be carried down, and at length, deposited in the
deeper parts of this sea bottom, where it will form a layer; and then,
while that first layer is hardening, other mud which is coming from the
same source will, of course, be carried to the same place; and, as it is
quite impossible for it to get beneath the layer already there, it deposits
itself above it, and forms another layer, and in that way you gradually
have layers of mud constantly forming and hardening one above the
other, and conveying a record of time.
It is a necessary result of the operation of the law of gravitation that the
uppermost layer shall be the youngest and the lowest the oldest, and
that the different beds shall be older at any particular point or spot in
exactly the ratio of their depth from the surface. So that if they were
upheaved afterwards, and you had a series of these different layers of
mud, converted into sandstone, or limestone, as the case might be, you
might be sure that the bottom layer was deposited first, and that the
upper layers were formed 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