CHAP. VIII. 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 

 terrestrial 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 

 was once situated. In that elevated alluvium the remains of 

 a tertiary mastodon and other quadrupeds of like antiquity 

 are embedded. 



If the Menchecourt beds had been first formed, and the 

 valley, after being nearly as deep 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 movement, 

 especially if it were intermittent, and interrupted occasionally 

 by long pauses, would very well account for the accumulation 

 of stratified debris which we encounter at certain points in 

 the valley, especially around Abbeville and Amiens. But we 

 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 considerable abundance, in 

 the drift both of higher and lower levels above Abbeville. 

 Had there been a total absence of all organic remains, 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 decomposing causes; but the post- 

 pliocene and implement-bearing strata can be shown by 

 their fossils to be of fluviatile origin. 



K 2 



