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