278 LECTUEE X. 



greater than the upper limit of the fall of the navigable rivers, 

 and that this fall even exceeds that of the Isere, Arve, &,nd 

 Bruche in the Yosges, where these rivers, in the vicinity of 

 their sources, flow with great rapidity, causing great devasta- 

 tions. All that is required is a heavy fall of snow or rain for 

 the waters to effect similar devastations upon the undulating 

 and loosely connected stony platforms of Picardy. Who could 

 thus undertake to determine the limits of the greatest effects 

 of this kind, which might have been produced on the environs 

 of Abbeville since the stone period ? 



"■ It has been jDarticularly pointed out that the sandbank of 

 Moulin-Quignon is older than the peat of the banks of the 

 Somme. This gravel deposit seems indeed to date from the 

 stone period, whilst the peats of northern France are partly 

 more recent than the Eoman roads. If this be true, it is easily 

 understood how the bones of the elephant and the rhinoceros 

 may have undergone the removal which produced these and 

 other deposits. They were then less petrified and less fragile 

 than now ; it remains nevertheless true, that the deposit at 

 Moulin-Quignon, as well as the peats, have been formed by 

 still acting causes, and belong, like the peat, to the present 

 period. 



" This gravel bank is composed of such variable deposits, 

 which have been formed and are still forming on the surface of 

 the earth by the agency of atmospheric influences, and which 

 I designate by the name of alluvium of the slopes, in contra- 

 distinction to the fluviatile alluvium which forms the planes of 

 the valleys. The alluvium on the slopes is very abundant in 

 the north of France on account of the composition of the ter- 

 tiary strata covering the chalk, in the mass of which the undu- 

 lations of the ground enter. 



'' The alluvia on the slopes are forming daily during every 

 fall of rain ; some are formed in the garden of the Luxemburg, 

 where the sand is scattered upon the paths, expressly as it were, 

 to produce this little phenomenon. The alluvium on the slopes, 

 that in the valleys, and the peats, are to be considered, in their 

 totality, essentially synchronous. 



" I will not enter into further particulars, but wait until the 



