EVIDENCE OP DENUDATION. 219 



then a slightly silicated limestone ; and that the present 

 purely calcareous character of the chalk is due to the 

 separation of the silica by segregation. We have, however, 

 still to account for the relatively large amount of soluble 

 silica present in the cretaceous rocks, since this is far in 

 excess of what would have been brought down by most 

 rivers of the present day, in the waters of which the 

 amount of this substance is almost infinitesimal. It has 

 been suggested that the unusual supply may have been 

 afforded by the cretaceous rivers being largely fed by sili- 

 ceous springs ; but although this may have been one factor 

 in the case, a more probable theory is that the drainage 

 area supplying the cretaceous sea with sediment was 

 largely composed of decomposed felspathic rocks, in which 

 the amount of this silica would have been amply sufficient 

 to have furnished the quantity present in the chalk. 



It is, however, not only with regard to its mode of 

 origin that flint is of more than ordinary interest. 

 Being an excessively hard substance, it is one exceedingly 

 difficult to be worn to powder by the action of water, and 

 the flint-gravels of the valleys of the south of England, 

 as well as the beaches of our southern coasts, and the 

 numerous Tertiar^ deposits composed of flint pebbles, 

 remain to us as silent witnesses of the vast denudation of 

 the upper chalk which has taken place in this country. 

 Remembering that the proportion of flint to chalk is only 

 from four to six per cent., and also bearing in mind that 

 all the flints in our gravels have been considerably reduced 

 in size by the action of water, we may fairly say that 

 every cubic yard of pure flint gravel represents the 

 removal of at the very least twenty cubic yards of pure 

 chalk; and to this we have to add all the lower chalk 

 which has been denuded without leaving any solid residue. 

 Moreover, when we recollect that this denuding process 

 has been going on ever since the Eocene period, and that 

 our river gravels only represent a small portion of the 

 flints left by the denudation of the chalk during the latter 

 part of this protracted period of time, we may gain some 

 faint conception of how enormous this denudation must 

 have been. 



