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.