polemics in opposition to the belief in former continental glaciation, which almost every one now considers definitely established, though after a hard fight.
It does not seem necessary at the present time to undertake to show how the glaciers did this, nor to prove that they could do it when the evidence is so clear that they actually did do it. Suffice it to say, that if glaciers smooth, scratch and pluck the rocks over which they pass, as every one knows they do (Fig. 13), it requires only a sufficiently long continuation of this action to lower valleys to any extent up to the time when they cease to further smooth, scratch and pluck. A century ago it seemed to many observers that at the slow observed rate of recession of Niagara Falls it was impossible to explain
the seven miles of gorge as a result of this process. No one now doubts this explanation of the Niagara gorge; and it is not doubted that the Colorado Canyon has been formed by slow sawing into the strata, like that which the river is now engaged in, but continued through a long period of time. An application of the same principle—a slow rate of erosion working for a long period of time—is all that is necessary to understand profound glacial erosion, once it is granted that glaciers