LAKES. 
247 
1306, on the Saleve 1330. This gives a slope of 
2J-2 to 3 per cent only. Now in the present Swiss 
glaciers the slope is about 6 per cent. That of the 
glacier of the Aar, which is the least inclined, is 
5 per cent. No doubt the greater the glaciei the 
less is the inclination at which it can move. Still a 
slope of 3 per cent would seem quite inadequate. 
If, however, we suppose that the Alps had a relative 
greater altitude of say 1000 metres the difficulty 
would be removed, and the glacier would have a 
sufficient fall. 
These and other considerations have led gradually 
to the opinion that while the valleys occupied by the 
Swiss lakes were mainly excavated by running water, 
the lakes themselves are due to changes of level 
which have raised parts of the valleys as compared 
with the river courses nearer the mountains. 
Prof. Heim has suggested that the compression 
which elevated the Swiss mountains, and piled, as we 
have seen {ante, p. q i), more than double the original 
weight on this portion of the earth’s surface, led to 
the formation of the great lakes. The mountain 
mass thus concentrated on a comparatively small 
area would from its enormous weight tend to sink 
somewhat into the softer magma below, which of 
course would have had in this respect the same 
