464 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [^pi** -5, 



higher relative level than the lower, yet the higher series are always 

 the older, and the lower the more modern ; and we have thus the 

 ordinary superposition of new over old strata supposed to be reversed. 

 This difficulty is considered by most geologists to have been sur- 

 mounted by Mr. Prestwich's arguments, and his classification of the 

 gravels has been generally accepted. My own opinion is, that .the 

 evidence on which Mr. Prestwich's theory is based is insufficient ; at 

 the same time, I fully admit the careful and accurate manner in 

 which he has stated his views. 



Present inquirers have the great advantage of having before them 

 the accurate sections prepared by Mr. Prestwich with great skill and 

 labour. Mr. Prestwich describes the drift-beds of the valley of the 

 Somme as the opposite extremities of a series of gravels deposited in 

 a period of time so vast that during its continuance extensive val- 

 leys were formed and excavated out of solid strata, and the gravel- 

 beds afterwards deposited on their flanks and on their bottoms. 



The lower valley-gravels are more continuous than the upper ; and 

 their lower extremities touch the alluvium of the present rivers, 

 often on the same horizon. 



They contain organic remains resembling those found in the well- 

 known bone- caverns, including the bones of extinct Mammalia, 

 associated with flint weapons of rude workmanship. The upper 

 beds contain nearly the same organic remains and human imple- 

 ments as the lower, and are occasionally capped by tbick beds of 

 loess of freshwater origin. Both upper and lower valley-gravels 

 contain materials brought from a distance and deposited irregularly, 

 as if they were the feeble successors of the great diluvial deposits of 

 the glacial period which immediately preceded them, but which in its 

 typical form left no remains south of the Thames. 



Mr. Prestwich has described the present valley of the Somme as 

 having a transverse section of five hundred times the area of the 

 present water-channel, and other valleys of similar character as 

 possessing still greater proportionate size when compared with their 

 present water-channels. 



He supposes that during the elevation of the land the action of 

 torrents, ice, and snow removed large masses of stratified rock from 

 the valleys ; and in writing of the duration of the period of time in 

 which the upper and lower gravels might have been accumulated, 

 he observes: — " The next possible standard to measure the dura- 

 tion of the period of time in which the ujDper and lower gravels were 

 accumulated is the time required for the excavation of the valleys 

 themselves." I do not see that this supposition is confirmed by suf- 

 ficient evidence ; for, supposing the excavation of the valleys to have 

 occurred between the dates of the upper and lower valley-gravels, 

 we are obliged to allow not only a vast, but an apparently unneces- 

 sary amount of time for the accumulation of such incoherent and 

 often unstratified deposits. The lower valley-gravels at Grays, 

 Ilford, Menchecourt, <fec. &c., are thick, and must be the accumula- 

 tion of a long period ; for they contain remains in great abundance 

 of the large extinct Mammalia, deposited among fragile shells of 



