THE MOUNTAINS OF SWITZERLAND. 93 
erosion and denudation have stripped from the 
summits of the mountains! Fig. 26 gives a section 
across the Alps, and it will be seen that the section 
across the St. Gotthard substantially resembles that 
of Mont Blanc. 
Surprising, and even almost incredible, as this 
may at first sight appear, it becomes less difficult to 
believe when we remember that not only the great 
Miocene gravel beds which form the Central plain of 
Switzerland, but much of the deposits which occupy 
the valleys of the Rhine, Po, Rhone, Reuss, Inn, and 
Danube- — the alluvium which forms the plains of 
Lombardy, of Germany, of Belgium, Holland, and of 
South-east France consists of materials washed down 
from the Swiss mountains. 
It is calculated that at the present rate of erosion 
the Mississippi removes one foot of material from its 
drainage area in 6000 years, the Ganges above 
Ghazipur in 800, the Hoangho in 1460, the Rhone 
in 1500, the Danube in 6800, the Po in 750. Pro- 
bably therefore we may take the case of the Rhone 
as approximately an average, and this gives us, if not 
a measure, at any rate a vivid idea of the immense 
length of time which must have elapsed. 
The great plain shows comparatively gentle eleva- 
tions, which become more marked in the “Prealps,” 
