254 OEIGINAL AETICLES. 



similar in composition and contents, that we seem justified in assuming 

 it to have been at one time continuous ; and we may almost take the 

 section, Pig. 4, as representing generally a section taken anywhere 

 across the valley, only bearing in mind that through the action of 

 subsequent causes, the gravel and the beds covering it have been in 

 most cases removed. Nor is this a phenomenon peculiar to the 

 Somme. During our last excursion we visited many gravel pits 

 liolding a similar relation to the Seine, while Mr. Prestwich in his 

 recent communication to the Eoyal Society,* extends the same state- 

 ment to many other rivers in England and Prance, the greatest 

 height of the gravel above the present river level, varying however 

 in different cases. At St. Acheul and in several other places this 

 bed of gravel, Avhich for the future we "onll call the liigh level gravel, 

 is separated from the loio level gravel by a bare tract of the 

 underlying rock. We do, however, sometimes find beds at inter- 

 mediate levels, and must therefore consider the upper level, and 

 lower level gravels as the extremes of a continuous series, rather 

 than as strata separated by an intermediate and difterent condition 

 of the valley. 



The mammalia found in this upper level gravel are but few ; the 

 Mamuioth, the Hhinoceros tichorhinus, with species of Hos, Cervus, 

 and Eqnus are almost the only ones which have yet occurred at St. 

 Acheul, but beds of the same age in other parts of Prance have, in 

 addition, supplied us with remains of the Bear, of a species of Tiger, 

 of the Hycena spelcea, Cervus tarandus priscus, of a species of Dog, 

 of the Musk Ox, and the Jlipfopotmmis. The Mollusca however are 

 more numerous ; they have been identified by ]\ii'. J. Gr. Jeffreys, 

 who finds in the upper level gravel 43 species, all of them land or 

 freshwater fonns, and all belonging to existing species. It is hardly 

 necessary to add that these shells are not found in the coarse gravel, 

 but only here and there, where quieter conditions, indicated by a 

 seam of finer materials, have preserved them from destruction. Here, 

 therefore, Ave have a conclusive answer to the siiggestion that the 

 gravel may have been heaped up to its present height by a sudden 

 irruption of the sea. In that case we should find some marine re- 

 mains ; but as we do not, as all the fossils belong to animals which 

 live on the land, or inhabit fresh waters, it is at once evident that 

 this stratum not being subaerial, must be a freshwater deposit. 



But the gravel itself tells its even more than this: the river Somme 

 flows through a country in which there are no rocks older than the 

 chalk, and the gravel in its valley consists entirely of chalk flints and 

 tertiary debris.-f The Seine, on the other hand, receives tributaries 

 which drain other formations. In the valley of the Tonne we find 

 fragments of the crystalline rocks brought from the Morvan. J The val- 

 ley of the Oise is in this respect particularly instructive : " de Ma- 



* rroccedings. 1862. f Buteux, 1. c, p. 98. 



X D'Archiac, rrogres de la Geologic, p. 163. 



