LAKES. 
237 
Very slight changes might again greatly enlarge 
the lakes. For instance, if the narrow outlet of the 
Aar, somewhat below Brugg, were again closed, a 
great part of the central Swiss plain would be sub- 
merged. 
The problem of the origin of lakes is by no 
means identical with that of rivers. We have not 
only to account for the general depth of the valley 
this may be due to running water — but for the 
exceptional basin of the lake; running water produces 
valleys, it tends to fill up and drain lakes. 
To what then are lake basins due? 
It used to be supposed that many lakes were 
due to splits and fractures. I do not, however, know 
of any Swiss lake which can be so explained. 
We may divide Lakes into four classes: 
1. Lakes due to changes of level. 
2. Lakes of embankment. 
3. Lakes of subsidence. 
4. Crater lakes. 
In many cases, however, a lake may be due partly 
to one of these causes and partly to another, and for 
convenience of description they may be dealt with 
under eight heads: — 
1. Those due to irregular accumulations of drift; 
these are generally small and shallow. 
