Chap. V. AND DENUDATION. 245 



the powerful action of frost. The free access 

 of air will likewise be excluded. From these 

 several causes disintegration would be almost 

 arrested, if the overlying mould were to 

 increase much in thickness, owing to none or 

 little being removed from the surface.* In 

 my own immediate neighbourhood we have a 

 curious proof how effectually a few feet of 

 clay checks some change which goes on in 

 flints, lying freely exposed ; for the large 

 ones which have lain for some time on the 

 surface of ploughed fields cannot be used for 

 building ; they will not cleave properly and 

 are said by the workmen to be rotten, f It is 



* The preservative power of a layer of mould and turf is often 

 stown by the perfect state of the glacial scratches on rocks when 

 first uncovered. Mr. J. Geikie maintains, in bis last very inter- 

 esting work (' Prehistoric Europe,' 1881), that the more perfect 

 scratches are probably due to the last access of cold and ihcrease 

 of ice, during the long-continued, intermittent glacial period. 



f Many geologists have felt much surprise at the complete 

 disappearance of flints over wide and nearly level areas, from 

 which the chalk has been removed by subaerial denudation. 

 But the surface of every flint is coated by an opaque modified 

 layer, which will just yield to a steel point, whilst the freshly- 

 fractured, translucent surface will not thus yield. The re- 

 moval by atmospheric agencies of the outer modified surfaces 

 of freely exposed flints, though no doubt excessively slow, to- 

 gether with the modification travelling inwards, will, as may be 

 suspected, ultimately lead to their complete disintegration, not- 

 withstanding that they appear to be so extremely durable. 



