RECAPITULATION. . 311 



about twenty-five feet above the sea-level.* No doubt this 

 change of level has had an important bearing on the excava- 

 tion of the valley, but I cannot quite agree with Mr. Prest- 

 wich as to the effect which it has produced, f 



At length the excavation of the valley was completed ; the 

 climate had gradually become more like our own, and whether 

 from this change, or whether perhaps yielding to the ir- 

 resistible power of man, the great Pachydermata became 

 extinct. Under new conditions, the river, unable to carry 

 out to sea the finer particles brought down from the higher 

 levels, deposited them in the valley, and thus raised some- 

 what its general level, checking the velocity of the stream, 

 and producing extensive marshes, in which a thick deposit of 

 peat was gradually formed. We have, unfortunately, no trust- 

 worthy estimate as to the rate of formation of this substance, 

 but on any supposition the production of a mass in some 

 places more than thirty feet in thickness must have acquired 

 a very considerable period. Yet it is in these beds that we 

 find the remains of the Neolithic or later Stone period. 

 From the tombs at St. Acheul, from the Roman remains 

 found in the superficial layers of the peat, at about the 

 present level of the river, we know that fifteen hundred 

 years have produced scarcely any change in the configuration 

 of the valley. In the peat, and at a depth of about fifteen 

 feet in the alluvium at Abbeville, are the remains of the 

 Stone period, which we believe from the researches in Den- 

 mark and Switzerland to be of an age so great that it can 

 only be expressed in thousands of years. Yet all these are 

 subsequent to the excavation of the valley ; what antiquity, 

 then, are we to ascribe to the men who lived when the 



* The higher level gravels in some croachment of the sea on the land, and 



places fringe the coast at an elevation the consequent intersection of the old 



of as mnch as one hundred feet ; this river heds at a higher level, 

 phenomenon, however, I should be dis- f Phil, Trans. 1864, p. 297. 

 posed to refer principally to an en- 



