286 Mr. Lyell on the Strata of the Plastic Clay Formation, ^c. 



loam and fine sand. The size of the valleys is in general in proportion to that 

 of the streams flowing- in them, and their excavation appears referable for the 

 most partj if not entirely, to the long continued agency of these streams. It is 

 impossible not to draw this conclusion, when we observe the power exerted at 

 present by several small rills, which have divided the cliffs in some places 

 where their height is greatest, by perpendicular fissures or narrow ravines, the 

 sides of which still remain vertical, because the water constantly undermines 

 them and removes the loose sand, clay, and gravel. 



