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low declivity developed by varigradation; and at such localities, accordingly, glacial lakes, the submerged rock-basins characteristic of fjords, and other evidences of energetic ice-action remain after the melting of the ice.)

It follows, then, that the third feature of glacial cañons may result simply from glacial occupation of water-cut cañons; and since in the common mountain region from which the glaciers have completely disappeared the irregularities of gradient peculiar to such cañons are not greatly intensified, while glaciated rock-basins are comparatively rare and of slight depth, it equally follows that the occupation was only temporary, and the sum of glacial erosion relatively inconsiderable.

The immediate effect of the origin of a tributary cañon in a developing drainage-system is the exposure of a greater length of canon-wall to degradation; from which effect in turn results (under certain conditions of homogeneity of terrane and uniformity of altitude in the region, and hence of repeated bifurcation and wide dispersal of the branches of the nascent tributary) the formation of an amphitheatre opening into the main cañon. Then, after the considerable development of the tributary, its disposition wil be, as shown by Warren,^[1] to dam the main stream and diminish the declivity above its confluence; whereby lateral correasion will increase at the expense of vertical corrasion there. Thus, but increased lateral corrasion the amphitheatre will ever tend to expand within certain limits immaterial in this discussion. Such amphitheatres, exhibiting the tortuous outlines characteristic of fluvial erosion, have been well illustrated by Dutton,^[2] and are common features in many mountain regions.

If now a glacier enter and fill such an amphitheatre, its rate of flow and similarly its rate of erosion on the given area will be