DIRECTIONS OF RIVERS. 
233 
Age of Rivers. 
It follows from these considerations not only that 
some Swiss rivers are of comparatively recent origin, 
while others date back to very great antiquity, but 
that different parts of what is now considered a 
single river are of very different ages and have a 
very different history. 
The southern part of the Central Alps are sup- 
posed to have been first raised above the waters, 
and to have formed an Island in Eocene times, to 
which therefore some of the head-waters date back. 
It is, however, clear that the rivers crossing the 
Miocene deposits of Central Switzerland cannot have 
commenced until after the Miocene strata had been 
raised and become dry land. In fact the upper 
parts of the Reuss and the Aar probably represent 
the rivers which brought down the great masses ot 
Miocene gravel which now form the lowlands of 
Switzerland, and through which they subsequently 
cut the lower parts of their courses. These therefore 
must necessarily be of much less ancient origin; but 
even these valleys were as a rule excavated to their 
full depth before the Glacial period, and must there- 
fore be of immense antiquity. 
