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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



detritus of our earliest British Tertiary age. The superficial debris to 

 which I have wished to restrict attention is carefully to be distinguished 

 from that more ancient form of drift ; and this has been done by 

 showing that Tertiary formations have themselves been broken up by 

 the disruptions alluded to, and their debris mixed up with the drift 

 in question. 



The prominent object I have had in view was to impress upon the 

 mind, how a period of perfect quiescence and ordinary causation along 

 the southern shores of England, as proved by the round water-worn 

 pebbles of the Brighton shingle-bed, was suddenly succeeded by a 

 tumultuous accumulation of angular drift, which, having necessarily 

 resulted from violent action, can, I think, be only well explained by 

 reference to some of the last powerful fractures of the Chalk and over- 

 lying Tertiary strata. Now, these sharply-contrasted physical features 

 of quiescence and turbulence pertain albeit to one and the same epoch 

 of life. The quiet sea-beach was accumulating, and the same great 

 animals were living, with marine species of shells still existing, when 

 a catastrophe occurred which destroyed many of the former. 



On reference to the Wealden as a whole, and where it extends into 

 France, I am indebted to my friend Mr. Prestwich for indicating a fact 

 of great interest, hitherto unnoticed, and on which I hope he will give 

 us his views in extenso*. In examining the coast-cliffs of France to 

 the south of Calais and its dunes, he found at Sangatte, which lies in 

 the eastern prolongation of the phsenomenon described at Folkestone 

 and Dover, two deposits exactly resembling those of Brighton, i. e. 

 an underlying water-worn beach and a superior pile of sharply-frac- 

 tured tumultuous drift. 



I wish therefore that the eminent naturalists who have given us 

 such good grounds for believing j- that England must have been 

 separated from France since the period of the great diffusion of plio- 

 cene animals, may see, as I do, in the physical facts cited, the proofs 

 that, in the region under consideration, many at all events of these 

 creatures were destroyed by the sudden and violent operations I have 

 described, and may admit that they did not all pass out of existence 

 by ordinary and gradual causes. 



It would, however, be erroneous to suppose, that all the fragments 

 of the drift of this period are in the same angular condition as that 

 of Brighton, Lewes, and the South Downs. The result has been 

 somewhat different in respect to the drift of the same materials which 

 were thrown off northwards from the North Downs into the depres- 

 sion which is now the valley of the Thames. In that tract many of 

 the flints are more water-worn, as may be well seen in Hyde Park, 

 Kensington Gravel Pits, and numberless spots on both banks of the 

 Thames, as well as under London itself. This difference in the form 

 of the same materials might be accounted for by supposing that where 

 the flints did not proceed from any lines of fracture, but simply from 

 the slopes of the chalk, they would necessarily be less fractured. 

 Again, we may imagine that the detritus shed off north-eastwards 



* See Mr. Prestwich's Paper, read June 25, 1851, supra, p. 274 et seq. 



t See particularly Owen's History of British Fossil Mammals (Introduction). 



