192 



THE GEOLOGIST. 



in flood time, grounding on the brick-earth and pushing a portion of it 

 into the underlying beds of sand, w here, as the ice gradually melted, it 

 would be left, caught, and squeezed in by the sand pressing itself into 

 place again. 



The two classes of evidence are, therefore, conformable. It is in har- 

 mony also with the existence of the large beds of brick-earth or loess 

 overlying the gravel, and which is, doubtless, the deposit of the old river 

 during floods, usual in a severe climate at the time of the melting of the 

 winter snows. The winter climate may probably have been as rigorous as 

 that of Northern Eussia or Northern Canada. Such a climate would not 

 be any bar to the presence of man, whose works are found in these old 

 shingle beds. It is true that none of his remains have yet been found in 

 these deposits, but they are found in caves of the same age. The abun- 

 dance of animal remains is the almost inevitable consequence of a country 

 subject to great river-floods, by which large numbers of animals are always 

 destroyed and swept down; man, on the contrary, guards against such 

 risks. Along the Northern American rivers of the present day, although 

 the remains of the buffalo and other animals occur in profusion, the re- 

 mains of man are scarcely ever met wich. There is every reason to expect 

 that this further and desirable proof may be forthcoming at no long 

 distance of time. 



Lastly, the speaker stated that the present river Somme only carries 

 down fine silt and mud, whereas the old river transported large masses of 

 coarse shingle ; therefore, it is to be inferred that the old river was one of 

 much greater power than the present one. During floods especially its 

 power must have been very great ; w ith greater transporting power the 

 river would possess greater excavating power ; at the same time the dis- 

 integration of rocks, especially such soft rocks as the chalk of this district, 

 produced by severe cold, combined with the effects of ground ice lifting 

 up from the bed of the river large quantities of the shingle, would hasten 

 the deepening of the valley. As it deepened, terraces of shingle have 

 been left at places on the slopes. It may be difficult to imagine a river 

 with so limited a collecting ground filling a valley a mile wide, but this 

 the speaker supposes to have been the case only during floods, and that 

 the ordinary channel of the river w as very much smaller. He instanced 

 a case in India where Dr. Hooker mentions a river which was only eighty 

 yards wide when he crossed it, but which, after the rains, covered a 

 channel three miles wide, and ran ten to twelve feet deep. I he melting 

 of the snow in the spring produces the same result in arctic regions as 

 heavy and continued rains in southern regions. 



Mr. Prestwich next exhibited a diagram to show what he conceived to 

 be the different phases of the phenomena, from the period w hen the beds 

 of St. Acheul were formed, until the valley assumed its present form and 

 dimensions. The plan, which was formed of a series of superimposed sec- 

 tions, showed — 



1. The old river during the deposition of the shingle and sand banks of 

 St. Acheul. — In this the bed of the river was occupied with large shingle 

 banks, which were left dry during the time the river was low. Mr. Prest- 

 wich supposes these to have been resorted to by early man, in consequence 

 of the number of large flints they contained, for making flint implements 

 on the spot. This may be one of the reasons why they are so numerous 

 at St. Acheul, w hich was shown to be one of those old shingle banks pre- 

 served from that time. Ice-floes dropped large blocks of sandstone into 

 the shingle. A space shut off in part by a shingle bank w ould account for 



