M. GRAS ATTACK ON THE FLINT-IMPLEMENTS. 



283 



to lie quite distinct. The more ancient one, immediately overlying the 

 chalk, is essentially composed of light yellowish or brown flints, for the 

 most part rolled, disseminated through a whitish-grey calcareous sand. 

 The relative proportion of the sand and flints varies ; sometimes veins of 

 almost pure sand alternate with flints, or cover them. It is not uncommon 

 to find in the sand freshwater shells, almost intact, in spite of their fragi- 

 lity, — a fact which indicates a slow process of accumulation. Ferruginous 

 infiltrations from above have often stained the naturally clear colour of 

 this deposit. This diluvium has a very unequal thickness, owing to the 

 numerous erosions which it has undergone. It shows itself at St. 

 Acheul at a height of from thirty to forty metres above the Somme ; at 

 the sand-pits of Moutiers, at the western extremity of Amiens, it descends 

 all at once to the bottom of the valley ; finally, at Menchecourt, a suburb 

 of Abbeville, it passes beneath the turf-beds. It results from this, that 

 before the deposit of this transported bed, the Somme had already hollowed 

 out its channel in the bosom of the chalk, which is seen rising right and 

 left to a great height. The valley was even then deeper than it is now ; 

 it appears to have been entirely filled up at the time of the arrival of the 

 rolled flints. The second diluvial bed in the neighbourhood of St. Acheul 

 is an argillaceous-sandy stratum of a dark brown, of which the thickness 

 is usually from a metre and a half to three metres ; it is almost everywhere 

 dug for brickmaking. It shows usually at its base a thinnish layer of 

 angular flints disseminated through a brown earth, rather more sandy than 

 the rest of the mass. This argillaceous-sandy diluvium extends crosswise 

 at once over the lower clear grey diluvium and over the chalk ; it presents 

 all the signs of complete independence. Its deposition probably coincided 

 with the second excavation of the valley ; it is observed, in fact, at different 

 levels corresponding with those at which the Somme has successively 

 flowed before withdrawing itself to its present bed." 



In this account there are three topics which call forth observations. 

 1. There are (at least) two diluvial deposits. — There is nothing new 

 in this. Mr. Prestwich, one of the most inquiring and capable investi- 

 gators of the subject, and one of the strongest believers in^ the correct- 

 ness of Boucher de Perthes' assignment of the chipped flint-imple- 

 ments to the Gravel age, has already shown, that there is a " high- 

 level gravel" and a low-level or " valley-gravel ;" and in his papers 

 before the Royal Society has shown, also, why there are these depo- 

 sits, what are their relations to each other, the probable physical 

 and meteorological conditions under which they were deposited, and 

 their bearings in respect to the evidence of the flint-implements as 

 a proof of the antiquity of man. 2. That it is not iincomniGn to find 

 in the sand freshwater shells, almost intact, in spite of their fragilitif. 

 — There is nothing extraordinary in this. The wonder would be if 

 we did not find them. Mr. Prestwich has shown how much ice-action 

 had to do with the bringing down, during the early spring floods, of 

 the flints, rock-boulders, and other heavy materials, — probably often 

 also the bones of animals ; and if these heavier substances were frozen 



