M. GRAS' ATTACK 0>' THE FLINT-IMPLEMEXTS. 
283 
to be 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 ^loutiers, 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 tui-f-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 tlie neighbourhood of St. Acheul 
is an argillaceous-sand}^ 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 sho\A s usuall}' 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 Hoyal 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 uncommon to find 
in the sand freshwater shells^ almost intact, in spite of their fragility. 
— 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 
