DIRECTIONS OF RIVERS. 
213 
a condition. It seldom happens, however, that the 
case is so simple, and the lines of rivers offer many 
interesting problems, which are by no means easy to 
solve. 
As already mentioned ( atite , p. 174)1 the Swiss 
rivers follow two main directions, at right angles to 
one another, namely, S.W. by N.E. and N.W. by S.E. 
The first follows the strike of the strata. The ex- 
planation of the second is not so simple. The pro- 
bable cause, however, which has determined the two 
main directions of the Swiss rivers has been already 
suggested {ante, p. 181). 
The principal Swiss rivers must be of great 
antiquity. Some of the streams in the eastern and 
central parts of the Alps probably commenced even 
in Eocene times. The Nagelflue was brought down 
from the mountains by rivers which probably oc- 
cupied the upper parts of the valleys of the Aar, 
Reuss, etc. 
Nevertheless there have been great changes in 
the courses of the Swiss rivers. These are ascribable 
to four main causes: — First, it must be remembered 
that streams are continually eating back into the 
hills. In many cases they cut completely through 
them, and if the valley into which they thus force 
their way is at a higher level, they carry off the 
