240 ELEMENTARY GEOLOGY 
THE Errect oF DENUDATION 
The combined action of weathering and erosion is 
wearing down the surface of the land at a rate slow 
indeed, yet productive of decided effect in course of 
ages. If this action had proceeded uninterruptedly, 
the land would, ere this, have been worn down nearly 
to the condition of a plain; but as matter of fact, there 
is always operating an opposite set of forces tending to 
elevate the land. So the real result in these cases has 
been deep cuttings into the rocks, forming valleys 
where the conditions were favorable, and leaving hills 
between. The existence of soft rocks, or the location 
of a stream, has determined the lines of valleys, while © 
hills stand up above the general surface of the country, 
either because they are between the streams, or else 
because their site is determined by a hard rock. 
In the long ages through which these forces have 
been operating, not only have the lands been carved 
into hills and valleys, and the coasts changed again and 
again, but the general land surface has been slowly 
worn down; thousands of feet of rock have been de- 
cayed and moved to the seas; mountains have been 
planed down; volcanoes have dwindled away under the 
action of denudation; and the whole aspect of the con- 
tinents has undergone most profound change. 
