ACTION OF RIVERS. 
207 
the junction of the Rhone and the Valorsine below 
Geneva. These, however, will be destroyed as erosion 
continues. Surprise is sometimes expressed that 
Giants’ Caldrons occur where no stream now flows. 
But it is just to this fact that they owe their existence. 
If the river had not changed its course they would 
long since have been destroyed. 
Before closing this chapter I must say a few 
words about subterranean streams. These occur 
mainly in porous rocks, such as those of the Jura. 
The most considerable of these partly subterranean 
rivers is the Orbe, which rises originally in a little 
French lake, Les Rousses, traverses two others on 
Swiss territory, the Lake de Joux, and that of Brenet, 
and then disappears suddenly in the ground at the 
foot of a high cliff, reappearing again at a distance 
of 3 km. near Vallorbes. 
Summing up this chapter we may say that as 
soon as any tract of land rose out of the sea, the 
rain which fell on the surface would trickle down- 
wards in a thousand rills, forming pools here and 
there, and gradually collecting into larger and larger 
streams. Whenever the slope was sufficient, the 
water would begin cutting into the soil and carrying 
it off to the sea. This action would, of course, differ 
in rapidity according to the slope and hardness of 
