LECTUEE X. 265 



was tliat every brook gradually washed off tlie tertiary forma- 

 tion and transformed its harder parts into rolled pebbles. 

 Hence we only find the tertiary formation at a distance from 

 rivers, specially from the main stream of the Somme, upon the 

 plateau, and mostly covered by the old diluvium, a fatty clay 

 or brick earth, itself mostly derived from the destruction of the 

 tertiary formation, and forming- an extremely fertile bed about 

 five feet thick, which contains no fossils. Into this old diluvium, 

 as well as into the tertiary strata, and deep into the chalk, have 

 the streams and brooks dug- their beds ; and the valley in which 

 each of these streams flows, a valley of comparatively consider- 

 able width, is thus bordered on both sides by a chain of hills, 

 the slopes of which towards the stream consist of white chalk, 

 above which at some distance is spread out the fertile loam, 

 while beneath lie the sandy tertiary strata.. The bed of the 

 Somme, near Amiens, is nearly a mile in width, but enlarges 

 considerably from below Abbeville down to St. Valery. In 

 this river-bed, as well as in the neighbouring valleys, occur form- 

 ations which are manifestly more recent than the excavation 

 of the river bed, the tertiary strata, and the alluvial formation 

 of the platform. These formations within the old river valleys 

 claim our special attention, as they contain human remains. 



Kg. 86 Section of the Valley of tlie Somme, at Abbeville^ after Prestwich. 



S. Tlie river Somme. M. Sea-level. 1. Peat in the valley. 2. Subjacent 

 Letten. 3. Flint gravel reposing upon chalk. 4. Grey diluvium, with bones 

 and hatchets. 5. Calcareous loam or loess. 6. Brown clay and vegetable 

 sou. 7. Chalk. 



On the sides of the valley there are comparatively very slight 

 dejDOsits of rolled gravel, marl, sand and clay, forming two 

 different terraces, to distinguish which a practised eye is requi- 

 site. In the lowest terrace, from twenty to forty feet thick, there 

 is immediately beneath and upon the chalk a layer ten to fourteen 



