PT. irr. OR POISONED BY VEGETABLE GROWTH? 163 



of the road, as conipared with the ground tlirough 

 which it goes, will continue even after the road is 

 gravelled or stoned : because in proportion as the sur- 

 face of declivities is liard 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 undermining water 

 acts more rapidly than disintegration and ivash, and 

 clears away all that falls. Abstract this power, and 

 the chfF has a tendency 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 

 off its brow will lie at the foot of the cliff as a talus 

 or shelving bank ; and what was the mid clifT gra- 

 dually becomes the sole cliff. But this will eventually 

 disappear into one slope. If the top of the clifF is 

 table-land, or slopes from it, the cliff will waste much 

 more slowly by disintegration, and the action of the 

 elements But supposing such a clilf to be all of the 

 same material, I think the brow has a tendency to 



M 2 



