AND THE AVON GOKGE. 
177 
this, keeping its cutting edge parallel to the surface, we shall cut 
a deep notch-like groove in the slab. The groove will be of 
equal depth in the soft slate and in the hard quartzite ; for the 
depth to which the file can cut into the mass as a whole, is 
limited bj the depth to which it can cut into the hardest band. 
Now, laying aside our file, let us apply our sandpaper. After 
scouring vigorously for a short time, we shall find that, in the 
softer slate, the material has been worn away on either side of 
the groove to such an extent that the groove has become a wide 
depression, with sides very gently sloping down to that which 
was the bottom of the notch-like groove. But in the quartzite 
band the case is different. Here the sandpaper has less effect. 
The groove is scarcely or not at all widened. The quartzite 
band, moreover, stands out as a ridge across our slate; for the 
sandpaper has worn away the softer slate on either side of the 
harder band. 
Now let us see how far this illustration serves to aid us in 
the comprehension of the action of general and special denuda* 
tion on a large scale. The action of a river is like the action of 
our file ; it cuts a deep notch, gorge, or ravine. It is true that 
the river-file has some lateral play; for streams not only cut 
downwards but also eat into their banks and thus cut sideways. 
And they do this the more the softer the strata through which 
they flow. Still this only somewhat widens the groove. And it 
remains true that the office of rivers is to cut deep trenches into 
the surface of the land. They are the agents of denudation 
along special lines. And just as the file is unable to cut deeper 
into the softer slate than into the harder quartzite, so is a river 
unable to cut deeper into softer than into harder rocks — that is, 
so long as the softer rocks lie further up stream than the harder 
rooks. A band of hard rock, in fact, limits the depth to which 
a river can cut its way down into all the strata which lie 
further up stream, no matter how soft they may be. 
