wave, loaded as it is with particles of sand and gravel as it breaks
upon the shore, does something towards the disintegrating process. And
thus, slowly but surely, the hardest rocks are gradually ground down to
a powdery substance; and the mud thus formed, coarser or finer, as the
case may be, is carried by the rush of the tides, or currents, till it
reaches the comparatively deeper parts of the ocean, in which it can
sink to the bottom, that is, to parts where there is a depth of about
fourteen or fifteen fathoms, a depth at which the water is, usually,
nearly motionless, and in which, of course, the finer particles of this
detritus, or mud as we call it, sinks to the bottom.

Or, again, if you take a river, rushing down 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