LAKES. 
2 43 
on the solid rocks, and the fine mud which is carried 
down by glacial streams. It is of quite a different 
character from river mud, being soft and impal- 
pable, while river mud is comparatively coarse and 
gritty. 
The diminution in the rapidity of motion of a 
glacier at the sides and near the bottom, which has 
been relied on as evidence that glaciers cannot 
excavate, shows on the contrary how great is the 
pressure. 
The question has been sometimes discussed as if 
the point at issue were whether rivers or glaciers 
were the more effective as excavators. But this is 
not so. 
Even those who consider that lakes are in many 
cases due to glaciers might yet admit that rivers 
have greater power of erosion. There is, however, 
an essential difference in the mode of action. Rivers 
tend to regularise their beds; they drain, but cannot 
form, lakes. As Playfair long ago pointed out* a lake 
is but a temporary condition of a river. Owing in 
fact to rivers, lakes are mere temporary incidents. 
The tendency of running waters is to cut through 
any projection, so that finally its course assumes some 
16* 
Playfair’s Works, vol. I. 
