Page:Quarterly Journal of the Geological Society of London, Volume 24.djvu/238

120 120 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

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