In connection with the discussion on the origin of hills and valleys, which has recently occupied the pages of the Geological Magazine, I would submit a few observations on some phenomena, in evidence of the great power of subaërial denudation, which seem scarcely to have been noticed with the prominence they deserve. I assume that the joint action of sea and river denudation is un-questioned, and that the main point under discussion is relative to which of these processes determined the final contour of the land. What I wish particularly to notice is that the forma of the whole land surface with some trifling exceptions (as lake basins, which appear to admit of special explanation) is merely a modification of the same principle of contour as the true river valley, exhibiting a system of watersheds by which almost everypart of the land is connected with the sea by adjacent land on a graduated series of levels lower than itself.