1865.] 



PRESTWICH SANGATTE BEACH. 



441 



It was desirable to determine, if possible, the origin 

 of these materials, as some interesting points of 

 ancient physical geography hinged upon this point. 

 I therefore again closely examined these sands 

 and the subordinate shingle, and succeeded in find- 

 ing a not inconsiderable number of subangular frag- 

 ments of chert of distinct Lower Greensand ori- 

 gin, together with a few pebbles of Lydian stone, 

 derived probably from some of the Portland beds, 

 and some pebbles of sandstone which might also 

 have come from the Boulonnais. I also found two 

 pebbles of a red granite, which may have been de- 

 rived from the granitic districts either of the Co- 

 tentin or of the Channel Islands. Still it is possible 

 that these granite pebbles may have been derived 

 indirectly from the Lower Greensand, which we know 

 to contain in places pebbles of granite and of other 

 older rocks, or from the more distant Boulder-clay or 

 beds of that age. I unfortunately lost my speci- 

 mens in returning to Calais before I had had the 

 opportunity of carefully examining them. The pre- 

 sence, however, of chert from the Lower Greensand, 

 and of pebbles of Lydian stone and of other rocks 

 from the Boulonnais district, suffices for our object, 

 which is to show the additional probability these 

 facts afford of the existence of a channel between 

 France and England anterior to the low- and pos- 

 sibly the high-level valley-gravels. For to the east- 

 ward of a line drawn between Cape Blanc-nez and 

 the South Foreland there is no coastline whence this 

 Lower Greensand chert could have been derived — 

 the area consisting of Tertiary strata — whilst a chan- 

 nel open to the westward woiild, after passing through 

 the belt of Chalk, intersect the Lower Greensand of 

 Folkestone and the Portland and Purbeck beds of 

 the Boulonnais, although that channel might have 

 been much narrower than the one now existing. At 

 the same time there is another explanation possible, 

 although I consider it much less probable. 



I have mentioned in my last paper in the ' Phil. 

 Trans.' that the old rivers of the Wealden area de- 

 bouched, as do those of the present day, outwards 

 into the Thames valley, but that they were of much 

 greater size and extent. I have evidence to show 

 that in the very centre of the Wealden area there 

 are remarkable cases of vaUey- excavation and old 

 river-action. I have even there found beds of high- 

 level gravel. Now these rivers, in their passage 

 across the Lower Greensand, always took up con- 

 siderable quantities of chert and ragstone fragments, 





't^ 



^ 



.is I 



m: 





o 



