; 
4 
’ 
] 
109 
lower part, must have suffered more from them; so that the sides 
at any given height are farther apart than they are lower down 
nearer the stream, where they have not been acted upon so long. 
The exact section of the valley will however depend upon the 
geological structure of the country through which it is formed, and 
the inclination of the river traversing it. If the rocks are hard, 
the width and depth of the valley will not be so great as if the 
rocks were soft; and other things being equal, its depth will be 
greater if the bed of the stream be steep than if it were nearly 
level. Generally we find that narrow valleys are steep, their steep- 
ness—and consequently the velocity of the streams flowing through 
them—being inversely proportional to their breadth; that is to 
say—the steeper they are, the narrower they are comparatively, 
other things of course being equal. The explanation of this is 
very simple. The erosive power of a stream is proportional to its 
velocity ; if this increases above a certain point, the action of the 
stream on its bed will be greater than that of the atmosphere and 
other kindred forces on its banks, the consequence of which will- 
be that the valley will be enlarged at a greater rate downwards 
than sidewise, so that it will eventually become deep and com- 
paratively narrow. On the other hand, if the inclination of the 
stream be very slight, its denuding action downwards will be less 
than that of the forces acting on the sides, so that the width of 
the valley will increase more rapidly than its depth. As a conse- 
quence of this, the valley will become, in the long run, broad but 
comparatively shallow. 
To some, probably, there may seem to be a great want 
of efficiency in these subaerial forces to effect a work so enor- 
mous as that with which they are credited, but is it so? No 
doubt so long as we were content to accept unquestioned Arch- 
bishop Usher’s figure of 6000 years as the age of the world, there 
might be a difficulty in reconciling such an enormous effect with a 
cause apparently so inadequate. But now that geologists—by 
j thinking for themselves, and by earnest study of the Book of Fact 
—have learned that it is millions of years since our earth began 
its circling flight through “the lucid interspace of world on world” 
