Mr. Webster on the Strata lying over the Chalk. 235 



deep water It may In some degree be produced by the action of 

 the tides ; but it is by the irresistible force of the billows and 

 breakers that it proceeds with the utmost rapidity. Hence all 

 geologists, in examining the history of the strata, have considered 

 rounded pebbles as proofs of the existence of land elevated above 

 the water. 



At this remote period of the earth, when the outline of the 

 coasts were, as now, deeply indented by gulphs and bays, but 

 whose forms and situations had but little if any correspondence 

 with the present, great changes must have taken place by the 

 gradual action of the sea ; and vast accumulations of pebbles of 

 different materials, formed by attrition, would be thrown upon 

 the shores. 



In the ancient Parisian gulph this phenomenon is distinctly pointed 

 out ; nor are similar appearances wanting to demonstrate the action 

 of the same causes in the Isle of Wight and London basins. 



We have seen that in Alum bay there are layers of rounded 

 pebbles In the vertical sand strata, which must have been formed 

 prior to the subversion of the chalk, and before the deposition of 

 the London clay. Similar rounded flints, and often of a whitened 

 appearance, are found In the sand strata at the bottom of the blue 

 clay in various parts of the London basin. 



Ail these belong to the most ancient flint pebbles, formed by the 

 same sea that deposited our blue clay and the calcaire grossler of 

 France. 



The thick bank of flint pebbles at Woolwich and Blackheath, 

 separated from the chalk only by sand, appears like the section of 

 an ancient beach, it being not far distant from the shores of the 

 ancient marine gulph or bay which now forms the London basin. 

 The fossil shells which we see in such numbers among these peb- 



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