120 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



very different from any now to be met with in these latitudes ; there- 

 fore there is no /)ri??ia /aae improbability in supposing a pluvial 

 period even of longer range in time than the glacial epoch. 



The existence of such a pluvial period is demonstrated by the 

 size, constitution, and level of the fluviatile gravel and loess depo- 

 sits at Amiens and other well-known localities. Large rivers cer- 

 tainly existed to a later date than the glacial period, as they formed 

 such large terraces of loess over the glacial gravels. If we 

 were to judge of the age of these later deposits, such as the loess 

 escarpments at Amiens and Clapton, by their modern appearance 

 and by their being unaltered by weather and not cut into by 

 streams, we should place them almost in the historical period. The 

 Amiens sections of loess accord with those of the Ehine and other 

 rivers. The difference between this loess deposit at Amiens and 

 the present warp of the Somme ought to be an index of the rainfall 

 in the pluvial period, when the loess was deposited, as compared 

 with that faUing at the present time ; and we may look at these 

 gravels and loess beds as registering rain-gauges. 



In the same manner, the comparative rainfall at the epoch of the 

 deposition of gTavels might be estimated approximately by com- 

 paring the dimensions of the blocks of Gres and large flints moved 

 by fresh water in the gravel-period with the size of the materials 

 moved in the same valleys at the present time. 



The existence of a glacial period almost necessitates that of a 

 pluvial period, commencing prior to the glacial, and continuing 

 after it, occupying a region south of that occupied by the ice and 

 snow. 



We should have had no cause for surprise if, when the theory 

 of a period of ice and snow in these latitudes was fii^st broached, 

 the probability of a rainy period south of the Thames had been 

 also deduced from a consideration of the effects of so large a mass 

 of ice and snow on the country and atmosphere bordering on the 

 ice-land, but possessing a warmer climate. 



We have the evidence in almost all wet valleys of the river 

 merely occupying a small groove cut in the ancient valleys, which 

 valleys I believe were shaped to their present configuration in such 

 a rainy period as I have inferred. Every wet valley has a number 

 of dry valleys opening into it, which bear the marks of having been 

 shaped by water and continual showers during the pluvial period. 



The points of difference between other authors who have wi'itten 

 respecting the Somme Valley and myself are as follows : — 



In the appendix to llr. Prestwich's paper in the ' Philosophical 

 Transactions,' M. Pinsard gives the height of the railway at La 

 Neuville as 83 feet above the mean tide at Havre. In the survey 

 made for me by M. Guillom, Principal Engineer of the Chemin de 

 Per du !N'ord, the height is 96 feet. ' (llr. Prestwich has marked 

 this same level as 76, in his drawing, plate 10. Phil. Trans. I860*.) 

 This is just 13 feet below the real height. Again, in ICr. Prestwich's 



* This is calculated from the mean tide at St. Yalery, -n-liich differs 7 feet from 

 that at Havre. 



