220 A FLAKE OF FLINT AND ITS HISTORY. 



Although the flint- gravels of our rivers afford some 

 estimate, however faint, of the denudation of the chalk 

 during the Pleistocene period, it would be quite incorrect 

 to assume that the flint pebbles forming the beaches of 

 our southern coasts present a record of the amount of 

 denudation which has taken place during the modern 

 period. We have already mentioned that a freshly-broken 

 flint presents a uniform blackish-blue colour throughout 

 its interior, and any flint pebble on the seashore which 

 had been recently derived from its native chalk would, 

 when broken, present a similar appearance. As a matter 

 of fact we shall find, however, that at least ninety per cent, 

 of such pebbles are stained yellow, brown, red, or black 

 internally, and since most of the flint fragments in many of 

 our older gravels are likewise similarly stained, we shall 

 have little hesitation in coming to the conclusion that our 

 modern sea-beaches are largely derived from the breaking 

 up of such old gravel beds, and the subsequent rounding 

 of these irregular fragments of flint into pebbles by the 

 action of the sea. The staining of the flints is of course 

 due to the large amount of ferruginous matter contained 

 in the gravels, and owing to the banded nature of the 

 original flint it frequently gives rise to an agate-like 

 appearance in the pebbles. Many of the pebbles in our 

 beaches are, however, derived from still older sea-beaches, 

 like the one now remaining at the southern extremity of 

 the Isle of Portland, while others, again, owe their origin 

 to the breaking up of the Eocene Woolwich and Beading 

 beds, which are largely composed of flint pebbles. Some- 

 times, indeed, fragments of these old beds also occur in 

 the river gravels, where blocks of the Hertfordshire con- 

 glomeratethe equivalent of the Woolwich and Keading 

 beds may be met with. 



We have thus abundant evidence of the exceeding 

 indestructibility of flint, and how it may go on from one 

 formation to another to tell, when rightly interpreted, the 

 various steps in the denudation of our country. 



In addition to being frequently stained internally, the 

 observer will also not fail to notice that all of the flints 

 in our river gravels have acquired a white or yellow 



