Imagine to yourselves the whole of St. Paul's churchyard filled
with oyster-shells, built up in a large square till they reached
half as high again as the top of the cathedral, then you will
have some idea of the amount of chalk carried invisibly past
Bonn in the water of the Rhine every year.

Since all this matter, whether brought down as mud or dissolved,
comes from one part of the land to be carried elsewhere or out
to sea, it is clear that some gaps and hollows must be left in
the places from which it is taken. Let us see how these gaps are
made. Have you ever clambered up the mountainside, or even up
one of those small ravines in the hillside, which have generally
a little stream trickling through them? If so, you must have
noticed the number of pebbles, large and small, lying in patches
here and there in the stream, and many pieces of broken rock,
which are often scattered along the sides of the ravine; and
how, as you climb, the path grows steeper, and the rocks become
rugged and stick out in strange shapes.

The history of this ravine will tell us a great deal about the
carving of water. Once it was nothing more than a little furrow
in the hillside down which the rain found its way in a thin
thread-like stream. But by and by, as the stream carried down
some of the earth, and the furrow grew deeper and wider, the sides
began to crumble when the sun dried up the rain which had soaked
in. Then in winter, when the sides of the hill were moist with the
autumn rains, frost came and turned the water to ice, and so made
the cracks still larger, and the swollen steam rushing down,
caught the loose pieces of rock and washed them down into its bed.
Here they were rolled over and over, and grated against each