116 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



In the same way no argument can be satisfactorily applied to the 

 formation of the Somme valley, without considering the relation of 

 the chalk-valleys generally, which resemble each other in so many 

 important respects, and are ahke in general features as well as in the 

 mineral composition of their strata and their superficial contents. 



On some other occasion, after the Society has heard my account 

 of some other valleys, and seen my measured sections, I shall ven- 

 ture to lay before them the views of the formation of the Somme 

 valley which I hold. I shall now only remark that the bottom of 

 the valley of Saveuse opens into the valley of the Cette, between 

 Montiers and Eenancourt, at a height of 92 feet above the sea, 

 and is 140 feet high above the sea near the Perme de Grace, and 

 187 feet above the sea near Saveuse, and that there are con- 

 tinuous terraces of loess, varying in form and elevation, sometimes 

 Tvith the configuration of the chalk-vaUey on which they repose 

 from one end to the other, but evidently the consequence of water 

 fiUing the Saveuse valley on its passage from Eerrieres to Montiers. 



2. The Gravel and Loess. — From the fine or coarse character of 

 the gravel, and from the thickness of the loess, we may infer some of 

 the physical circumstances which occurred at the period of deposition. 

 The loess is in some places sandy, and in others is a fine loam, but 

 it varies little in coarseness. At the same height above the river 



1 have observed great discrepancies in the thickness of the true 

 loess : thus, at a pit 200 yards east of the line N Q, 15 feet of 

 loess was well exposed in a new pit from which a good quality of 

 brick- earth was being removed and carried a long distance to the 

 brickfields at Montiers ; and I was informed there was a depth of 

 16 feet more before the gravel was reached. There was a gravel- 

 pit on the same level, a little to the west. On the edge of the 

 Saveuse valley, 400 yards south of 0, the loess was only from 1 to 



2 feet thick. At St. Acheul it was only 5 or 6 feet thick ; but 

 there were intermediate beds of marl and sand between the true 

 loess and the true gravel there. 



There seems to be a line of thicker brick-earth or loess run- 

 ning south and north between the lines P and N Q. This would 

 indicate that the water was more tranquil at that point. Such dif- 

 ferences in currents are very apparent in rivers at the present time ; 

 and the warp of our rivers approaches very nearly to the character 

 of loess. The fossiliferous gravels at St. Acheul extend to a height 

 of 70 feet above the river, or much higher than the correspond- 

 ing fossiliferous gravels in the valley of the Thames. The shells 

 are found in false-bedded fine sand, and not in clay, at St. Acheul, 

 and in precisely the same condition as at Crayford. 



The Cyrena shell-bed at Crayford, however, is only 38 feet above 

 the sea ; but both the St.-Acheul and Crayford gravels extend up- 

 wards and join the plateau beds, while they pass downward as far 

 as the river in both cases. 



The chalk is capped in some places with Tertiary sands at Cray^ 

 ford ; but the gravel lies on the concavities of chalk and saud quite 

 indifferently. 



