ACTION OF RIVERS. 
193 
laws, and a change at any part is continued both 
upwards and downwards, so that a new oscillation 
in any place cuts its way through the whole plain of 
the river both above and below. 
If the river has no longer a sufficient fall to 
enable it to carry off the materials it brings down, 
it gradually raises its bed (Fig. 48), hence in the 
lower part of their course many of the most celebrated 
rivers — the Po, the Nile, the Mississippi, the Thames, 
etc.. — run upon embankments, partly of their own 
creation. 
The Reno, the most dangerous of all the 
Apennine rivers, is in some places more than 30 
feet above the adjoining country. Rivers under such 
conditions, when not interfered with by Man, sooner 
or later break through their banks, and, leaving their 
former bed, take a new course along the lowest part 
of their valley, which again they gradually raise above 
the rest. 
Along the valley of the Rhone from Visp down 
to the Lake of Geneva there is often a marsh on 
one side of the valley, sometimes on both, the exist- 
ence of which may be thus explained. 
This is the second stage. 
Scenery of Switzerland. /. 
13 
