ACTION OF RIVERS. 
IQI 
evidence. Some of these gorges are left at times 
quite dry, and it is easy then to see that the rock is 
continuous from side to side. The tunnels on the 
§t. Gotthard line pass no less than six times under 
the Reuss, and there is no trace of a fault. 
It may, I think, be said that the theory which at- 
tributed these gorges to a split in the rock is now 
definitely abandoned. 
Of course, however, there are some cases in 
which the courses of streams have been determined 
by lines of fault and fracture. 
Second Stage. 
The second stage commences where the inclina- 
tion becomes so slight that the river can scarcely 
carry away the loose material brought from above, or 
showered down from the sides, but spreads it over 
the valley, in which it wanders from side to side, 
and which it tends continually to widen. Hence un- 
less they are confined by artificial embankments, 
such rivers are continually changing their course, 
keeping however within the limits of the same valley. 
The width of the valley moreover depends on its 
age, as well as on the size of the river and the 
character of the rock. 
If we imagine a river running down a regularly 
