TYLOll AMIENS GRAVEL. 105 



gravel or loess now to be seen there, and in this respect corresponded 

 with all other valleys in which Quaternary deposits of this character 

 are met with. 



Second, that the whole of the Amiens-valley gravel is of one for- 

 mation and of similar mineral character, and contains nearly similar 

 organic contents, the La Neuville, Montiers, and St.-Acheul gravels 

 being of the same age, and capped with a covering of loess also of 

 one age and mineral character, the whole deposit being of a date 

 not much antecedent to the Historical period. 



Third, that the gravel in the valley of the Somme at Amiens is 

 partly derived from debris brought down by the River Somme and by 

 the two rivers the Celle and the Arve, and partly consists of material 

 from the adjoining higher grounds, washed in by land-floods, — the 

 immense quantity of chalk present in the gravel having been derived 

 from the latter source. It is where the surface of the chalk is concave 

 that the gravel is thickest. 



Fourth, that the quaternary gravels of the Somme are not sepa- 

 rated into two divisions by an escarpment of chalk parallel to the 

 river as has been stated. They would have formed an exception 

 to other river-gravels if this had been the case. The St.-Acheul 

 gravels thin out gradually as they slope from the high land down 

 to the Somme, and they pass away into the Loess formation, — and 

 so also at Montiers. 



The Loess deposit, on the contrary, forms a distinct escarpment for 

 many miles along the Somme ; and this, I believe, is the bank of 

 the ancient river whose floods produced the St.-Acheul and Montiers 

 gravels. 



Fifth, that the existence of river-floods, extending to a height of 

 at least eighty feet above the present level of the Somme, is perfectly 

 proved by the gradual slope and continuity of the gravels deposited 

 by those floods upon the sloping sides of the valley towards the 

 Somme, and also by the Loess or warp, of similar mineral composi- 

 tion and colour, extending continuously over the wliole series of 

 gravels, and finishing with a well-defined bank near the present 

 stream. 



Beds of gravel, brick-earth, and loess, having an even sloping sur- 

 face from the escarpment of the sides of the valley down to the terrace 

 near the river-bank, are often to be observed near other rivers whose 

 channels bear the same proportion to their valleys that the Somme 

 river bears to its valley, and where gravel- and loess-deposits reach 

 to a height of 100 feet above the present river-levels. 



Sixth, that many of the Quaternary deposits in all countries, 

 clearly posterior to the formation of the valleys in which they lie, 

 are of such great dimensions and elevation that they must have been 

 formed under physical conditions very difierent from our own. They 

 indicate a Pluvial period, just as clearly as the northern drift indicates 

 a Glacial period. This Pluvial period must have immediately pre- 

 ceded the true Historical period. 



Since June 1866 I have visited Amiens several times, and com- 

 pared the gravels as accurately as I could, both as to situation and 



