Page:Popular Science Monthly Volume 54.djvu/490

472 water, there begin the attacks of the forces operating to destroy them. Hand in hand there go on growth and destruction. The two may keep an even pace; either may obtain the mastery. In the one case, lack of considerable elevation and flatness result; in the other, great altitudes may be attained. The rivers may cut their valleys downward as fast as the land rises, or the down-cutting may be relatively slower. In any case, after a given land mass has attained its greatest height above the sea, the larger rivers soon cut their channels down as far as river cutting is possible-—namely, to within a few feet of



sea level. With relatively rapid elevation, soft rocks, and large rivers, the resultant valley takes the form of a canon, examples of which are found along the courses of the Colorado and the Yellowstone Rivers (see Fig. 8). Valleys of this nature soon lose their steep sides by the action of weathering and all that this implies, and pass into a more open state, like that shown in Fig. 9.

These views have been selected in order that a comparison of this type of mountain structure may be made with that shown in Fig.