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MR* H. BURY ON THE DENUDATION OE [Nov. I9IO, 



were of exactly equal value, that the slow process of cutting back 

 the escarpment could continue for many miles without the forma- 

 tion of extensive subsequent valleys. Moreover, in certain ways 

 the conditions are peculiarly favourable for such formation : the 

 Chalk is hard though pervious ; but below it lies the impervious 



Gault, and below that again the pervious Lower Greensand, and rain 

 sinking into the latter would soon reach a saturation-point, deter- 

 mined by the level at which water could escape over the Gault. 

 Any river, therefore, which passed over the latter at an excep- 

 tionally low point would arain the underground waters of the 



