CHAP. viH. FLINT IMPLEMENTS IN GRAVEL NEAR AMIENS. 131 



the bones of fossil quadrupeds occur at all heights above the 

 present rivers from ten to one thousand feet, we observe the 

 teiTestrial fauna to depart in character from that now living 

 in proportion as we ascend to higher terraces and platforms. 

 We pass from the lower alluvium, containing the mammoth, 

 tichorhine rhinoceros, and reindeer, to various older groups 

 of fossils, till, on a table-land a thousand feet high (near Le 

 Puy, for example), the abrupt termination of which overlooks 

 the present valley, we discover an old extinct river-bed 

 covered by a current of ancient lava, showing where the 

 lowest level >vas once situated. In that elevated alluvium 

 the remains of a tertiary mastodon and other quadrupeds of 

 like antiquity are imbedded. 



If the Menchecourt beds had been first formed, and the 

 valley, after being nearly as deej^ and wide as it is now, had 

 subsided, the sea must have advanced inland, causing small 

 delta-like accumulations at successive heights, wherever the 

 main river and its tributaries met the sea. Such a move- 

 ment, especially if it were intermittent, and interrupted occa- 

 sionally by long pauses, w^ould very well account for the 

 accumulation of stratified debris which we encounter at cer- 

 tain points in the valley, especially ai-ound Abbeville and 

 Amiens. But Ave are precluded from adopting this theory 

 by the entire absence of marine shells, and the presence of 

 fresh-water and land species, and mammalian bones, in con- 

 siderable abundance, in the drift both of higher and lower 

 levels above Abbeville. Had there been a total absence of 

 all organic i-emalus, we might have imagined the former 

 presence of the sea, and the destruction of such remains 

 might have been ascribed to carbonic acid or other decom- 

 posing causes; but the post-pliocene and implement-bearing 

 strata can be shown by their fossils to be of fluviatile origin. 



