an that of the
Reuss itself, and the streams from them enter the main valley by rapids
or cascades. Again, rivers running in transverse valleys cross rocks
which in many cases differ in hardness, and of course they cut down the
softer strata more rapidly than the harder ones; each ridge of harder
rock will therefore form a dam and give rise to a rapid, or cataract. We
often as we ascend a river, after a comparatively flat plain, find
ourselves in a narrow defile, down which the water rushes in an
impetuous torrent, but at the summit of which, to our surprise, we find
another broad flat valley.

Another lesson which we learn from the study of river valleys, is that,
just as geological structure was shown by Sir C. Lyell to be no evidence
of cataclysms, but the result of slow action; so also the excavation of
valleys is due mainly to the regular flow of rivers; and floods, though
their effects are more sudden and striking, have had, after all,
comparatively little part in the result.

The mouths of rivers fall into two principal classes. If we look at any
map we cannot but be struck by the fact that some rivers terminate in a
delta, some in an estuary. The Thames, for instance, ends in a noble
estuary, to which London owes much of its wealth and power. It is
obvious that the Thames could not have excavated this estuary while the
coast was at its present level. But we know that formerly the land stood
higher, that the German Ocean was once dry land, and the Thames, after
joining the Rhine, ran northwards, and fell eventually into the Arctic
Ocean. The estuary of the Thames, then, dates back to a period when the
south-east of England stood at a higher level than the present, and even
now the ancient course of the river can be traced by soundings under
what is now sea. The sites of present deltas, say of the Nile, were also
once under water, and have been gradually reclaimed by the deposits of
the river.

It would indeed be a great mistake to suppose that rivers always tend
to deepen their valleys. This is only the case when the slope exceeds a
certain angle. When the fall is but slight they tend on the contrary to
raise their beds by depositing sand and mud brought down from higher
levels. Hence in the lower part of their course many of the most
celebrated rivers--the Nile, the Po, the Mississippi, the Thames,
etc.--run upon embankments, partly of their own creation.

[Illustration: Fig. 48.--Diagrammatic section of a v