ENGLISH CHALK DOWNS, 209 



out below it in the form of springs. That is why 

 clialk assumes such peculiar bossed and rounded 

 forms. Moreover, it gives rise to no rivers or 

 brooks, which cut themselves little valleys or 

 gorges in other strata; it is so porous that the 

 water which falls upnn it sinks in at once, carry- 

 ing down with it small quantities of tlie lime in 

 solution. Soil seldom forms upon the top; there 

 is nothing to retain it on such rounded slopes; it 

 gets washed away as fast as it grows, and is car- 

 ried down bv the surface drainajje into the combes 

 or hollows. These combes represent the parts of 

 the surface where the chalk hap])ened to be 

 originally softest, while the ridges are the liarder 

 and more compact places, often protected from 

 the constant waste by layers of Hint or concreted 

 lime-beds. 



AH the external peculiarities of the downs as 

 we know them nowadays thus depend ui>on their 

 primitive geological structure and their subse- 

 quent sculpture by wind and water. The short 

 turf with which they are covered depends upon 

 the scantiness and shallowness of the soil ; and 

 for the same reason trees are rare, growing for 

 the most i)art only in the combes, or, if on the 

 summit, then generally where old prehistoric 

 earthworks have permitted the accumulation of 

 deeper soil by preventing the constant downward 

 wash which generally takes place upon these 



