1866.] TTLOB — ^VALLEY-aBATEXS. 467 



action of weather dmring tke removal of the gravel in the valley above 

 and below it that the interval must be comparatively smaU since the 

 event in question took place, viz., since the valley previously filled with 

 gravel, sand, or mud was denuded, leaving what may be called high- 

 and low-level gravel in a remanie state on the flanks and the bottom- 

 Having repeatedly visited the vaUey of the Somme without being- 

 able to satisfy myself as to the correctness of the views entertained 

 by geologists regarding the extreme antiquity of the upper gravels^ 

 I wish to state that I attach great importance to the views so long 

 ago put forward, as to the valleys of post-Cretaceous age having been 

 filled with gravel in one geological period and cleared out and par- 

 tially refilled in a subsequent and, perhaps, very recent date*. 



The theory of the excavation of the French valleys between the 

 periods of the deposition of the upper and lower gravels was proposed 

 by Mr. Prestwich as the most probable explanation of the phenomena, 

 fhe impossibility of obtaining absolute proof of his hypothesis being 

 obviousf. 



All the evidence, en the contrary, which is forthcoming, T contend, 

 should rather cause us to admit the identification of the ages of the 

 Devonshire and Somme valleys ; for both sets of valleys are ancient, 

 they were afterwards wholly or partially filled with gravel, and the 

 gravel on the slopes and bottoms in each is remanie recently, the 

 levels of the new drift-deposits are similar, the character of the mam- 

 malian remains in each is strikingly alike, and raised sea-beaches 

 occur on the coast not very distant from the mouths of the valleys 

 in question. If the perforations of marine boring-shells could be 

 proved to occur in the valley of the Somme, the analogy would be 

 still clearer (but of this there is no evidence at the present time), as 

 it would then appear as if the deposition of these drift-beds might 

 have occurred almost within the historical period. 



It is, however, certain that both the low- and high-level gravels 

 with human implements and mammalian remains are more re- 

 cent than the Boulder-clay, for this is proved in the Hoxne section ; 

 and reasoning by analogy, all the other gravels in valleys and caves, 

 containing the same kind of organic remains associated with the 

 tools of uncivilized man, belong to an interval between the Boulder- 

 clay and historical periods. 



The raised sea-beaches containing only organic remains of spe- 

 cies of mollusca now living in their neighbourhood are the equiva- 

 lents in time of a portion, or the whole, of this period — as their sub- 

 aerial heads shade into modern deposits, while the base may, from 

 other reasons, be of much older date. Godwin-Austen separated the 

 lower portion of the Brighton beach from the upper, many years since, 

 in this manner. 



The conclusions at which I arrive are : — 



1st. That the valleys of the south-east of Devonshire and of the 

 north-east of France were excavated in remote geological periods, 



* Pengelly, Denudation of Eocks in Devonshire, p. 13. 

 t Prestwich, Phil. Trans, p. 253, 1864. 



