o cut
away the firmest bed rocks. Naturally the ease with which this carving
work is done is proportionate to the energy of the currents, and also
to the relative hardness of the moving bits and the rocks over which
they are driven.

So long as the rocks lie horizontally in their natural construction
attitude the course of the stream is not much influenced by the
variations in hardness which the bed exhibits. Where the strata are
very firm there is likely to be a narrow gorge, the steeps of which
rise on either side with but slight alluvial plains; where the beds
are soft the valley widens, perhaps again to contract where in the
course of its descent it encounters another hard layer. Where,
however, the beds have been subjected to mountain-building, and have
been thrown into very varied attitudes by folding and faulting, the
stream now here and now there encounters beds which either restrain
its flow or give it freedom. The stream is then forced to cut its way
according to the positions of the various underlying strata. This
effect upon its course is not only due to the peculiarities of
uplifted rocks, but to manifold accidents of other nature: veins and
dikes, which often interlace the beds with harder or softer partitions
than the country rock; local hardenings in the materials, due to
crystallization and other chemical processes, often create
indescribable variations which are more or less completely expressed
in the path of the stream.

When a land has been newly elevated above the sea there is often--we
may say, indeed, generally--a very great difference between the height
of its head waters and the ocean level. In this condition of a country
the rivers have what we may call a new aspect; their valleys are
commonly narrow and rather steep, waterfalls are apt to abound, and
the alluvial terraces are relatively small in extent. Stage by stage
the torrents cut deeper; the waste which they make embarrasses the
course of the lower waters, where no great amount of down-cutting is
possible for the reason that the bed of the stream is near sea level.
At the same time the alluvial materials, building out to sea, thus
diminish the slope of the stream. In the extreme old age of the river
system the mountains are eaten down so that the torrent section
disappears, and the stream becomes of something like a uniform slope;
the higher alluvial plains gradually waste away, until in the end the
valley has no salient features. At