344 Maw— On Watersheds. 



II. — On Watersheds. 



By Geoegb Maw, F.G.S., Etc. 



N connection witli 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 subaerial 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 form of the whole 

 land surface with some trilling 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 every part of the land is 

 connected with the sea by adjacent land on a graduated series of 

 levels lower than itself. 



Why is it that the surface is not irregularly undulating, exhibiting 

 a fan* proportion of its area in isolated depressions, surrounded on 

 all sides by land more or less higher than itself? and why are not 

 the valleys shut ofi^ into watersheds of defined area, terminating in 

 isolated lakes instead of almost invariably finding common outlets at 

 lower levels ? In other words, why is it that you can approach the 

 sea from any point of the earth's surface in an unbroken line of 

 descent ? 



The various complications of upheaval must have left the surface 

 with every variety of outline, including a fair proportion of isolated 

 depressions ; what then is the denuding power that has since stepped 

 in and almost obliterated them, and replaced the chaos of form by 

 the wonderfully uniform system of graduated levels that now, from 

 mountain top to sea coast, envelopes the whole land surface ? 



Is not the difiference between main channels, the recognised result 

 of river action, and the graduated undulation of the entire surface 

 one merely of degree ? and is not the cause assignable to the contour 

 of the principal valley also applicable to its tributaries, and to the 

 whole graduated series of inequalities leading therefrom up to the 

 very crest of the watershed ? 



The connection between this graduating system of contours and 

 the principle upon which subaerial denudation ought to act, seems so 

 natural that the onus prohandi of any more probable cause should 

 fairly lie with those who dispute the power of rain to effect the final 

 sculpturing of the land's surface. 



What can be more apposite than the apparent relation between 

 the delicate gradation of levels from the river mouths upwards to 

 the watershed boundaries, and the exactly proportionate concentra- 

 tion of water and consequent power of excavation doionwards from 

 the watershed lines to the river mouths. 



Marine denudation can only have had two modes of operation. 



