VIII RIVERS AND LAKES 287 
near Wallingford, but the escarpment. itself 
is still gradually retreating eastward. 
So, again, the Elbe cuts right across the 
Erz-Gebirge, the Rhine through the moun- 
tains between Bingen and Coblenz, the Poto- 
mac, the Susquehannah, and the Delaware 
through the Alleghanies. The case of the 
Dranse will be alluded to further on (p. 292). 
In these cases the rivers preceded the moun- 
tains. Indeed as soon as the land rose above 
the waters, rivers would begin their work, 
and having done so, unless the rate of eleva- 
tion of the mountain exceeded the power of 
erosion of the river, the two would proceed 
simultaneously, so that the river would not 
alter its course, but would cut deeper and 
deeper as the mountain range gradually 
rose. 
Rivers then are in many cases older than 
mountains. Moreover, the mountains are 
passive, the rivers active. Since it seems to 
be well established that in Switzerland a 
mass, more than equal to what remains, has 
been removed ; and that many of the present 
mountains are not sites which were originally 
