PI. nr. OR POISOXBD BY VEGETABLE GROAVTIl ? 160 



would be as bare of soil as the channels of torrents. 

 The channels of torrents and rivers prevent this effect 

 now ; they are nature's ditches and gutters : so that, in 

 this light, rivers may be regarded as a conservative, not 

 a destructive power. But rivers are mere labourers, or 

 accessories, in the affair. The wash of rain is the engi- 

 neer which has laid down the gradients of this pre- 

 ventive surface -drainao'e over the entire area of the 

 earth. The source of the valley is always much higher 

 up than the source of the river ; I mean, than the 

 spring source of the river: for the snow source or 

 glacier source, being both superficial sources, I consider 

 the same as the rain source of the valley. The river 

 has no power of making a valley above it ; but a torrent 

 of rain water has the power of scooping a valley below 

 it. Even on Salisbury plain^ which is comparatively 

 flat and covered with the closest greensward, these dry 

 valleys, or rather continuations of valleys, above the 

 heads of rivers exist : and that the cause which caused 

 them still works may be argued from the valleys being 

 richer in soil than the tops or sides of the hills, owing 

 to the gentle gradients of the bottoms of the valleys ; 

 for, other things the same, that is, in the same strata, 

 with tlie same vegetation, &c., one universal result of 

 the wash of rain is, that the degree of denudation of 

 soil will be directly as steepness. 



Some of these dry valleys have an almost imper- 

 ceptible slope ; and they might be perfectly level, and 

 yet be regular channels for the wash of periodical 



