Part III.] OR POISONED BY VEGETABLE GROWTH ? 
187 
or near declivities, very generally become con¬ 
duits' for the wash of rain. Hence our “ sunk 
lanes ” and “ sunk roads.” Without great care, 
this jinking of the road, as compared with the 
ground through which it goes, will continue even 
after the road is gravelled or stoned; because, 
in proportion as the surface of declivities is hard 
and imporous, the wash of rain and its power 
accumulate. So that, although declivities whose 
surfaces are soft are from this reason more 
easily abraded,—by their porousness, which is 
generally a consequence of their softness, they are 
to a certain extent protected from denudation. 
The deposit from the wash of rain on each 
side of all level roads is soon covered with 
growth; it then also catches the dust or deposit 
from the air, and, unless the pickaxe and shovel 
are constantly at work, the drainage of a road is 
soon choked up. 
As long as the sea or a river acts on the foot 
of a cliff, it remains a precipice, for the under¬ 
mining water acts more rapidly than disintegra¬ 
tion and wash , and clears away all that falls. 
Abstract this power, and the cliff has a ten¬ 
dency to conform to the slope, that is, to the 
wash of the hill above it, both at its brow and at 
its foot. For what is washed down the hill and 
