CHAP. viil. MARINE SHELLS AT MENCHECOURT. 123 



A geologist will perceive by a glance at the section that 

 the valley of the Somme must have been excavated nearly 

 to its present depth and width when the strata of No. 3 were 

 thrown down, and that after the deposits Nos. 3, 2, and 1 had 

 been formed in succession, the present valley was scooped 

 out, patches only of Nos. 3 and 2 being left. For these 

 deposits cannot originally have ended abruptly as they now 

 do, but must have once been continuous farther towards the 

 centre of the valley. 



To begin with the oldest, No. 3, it is made up of a suc 

 cession of beds, chiefly of freshwater origin, but occasionally 

 a mixture of marine and fluviatile shells is observed in it, 

 proving that the sea sometimes gained upon the river, whether 

 at high tides or when the fresh water was less in quantity 

 during the dry season, and sometimes perhaps when the land 

 was slightly depressed in level. All these accidents might 

 occur again and again at the mouth of any river, and give rise 

 to alternations of fluviatile and marine strata, such as are 

 seen at Menchecourt. 



In the lowest beds of gravel and sand in contact with the 

 chalk, flint hatchets, some perfect, others much rolled, have 

 been found; and in a sandy bed in this position some work 

 men, whom I employed to sink a pit, found four flint knives. 

 Above this sand and gravel occur beds of white and siliceous 

 sand, containing shells of the genera Planorbis, Limnea, 

 Paludina, Valvata, Cyclas, Cyrena, Helix, and others, all now 

 natives of the same part of France, except Cyrena fluminalis 

 (fig. 17), which no longer lives in Europe, but inhabits the 

 Nile, and many parts of Asia, including Cashmere, where it 

 abounds. No species of Cyrena is now met with in a living 

 state in Europe. Mr. Prestwich first observed it fossil at 

 Menchecourt, and it has since been found in two or three 

 contiguous sand-pits, always in the fluvio-marine bed. 



The following marine shells occur mixed with the fresh- 



