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.
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