similiar 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, Fig. 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 holding a similar relation to the Seine, while Mr. Prestwich in his recent communication to the Royal Society,[1] extends the same statement to many other rivers in England and France, 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, which for the future we will call the high level gravel, is separated from the low level gravel by a bare tract of the underlying rock. We do, however, sometimes find beds at intermediate 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 different condition of the valley.
The mammalia found in this upper level gravel are but few; the Mammoth, the Rhinoceros tichorhinus, with species of Bos, Cervus, and Equus are almost the only ones which have yet occurred at St. Acheul, but beds of the same age in other parts of France have, in addition, supplied us with remams of the Bear, of a species of Tiger, of the Hyæna spelæa, Cervus tarandus priscus, of a species of Dog, of the Musk Ox, and the Hippopotamus. The Mollusca however are more numerous; they have been identified by Mr. J. G. Jeffreys, who finds in the upper level gravel 43 species, all of them land or freshwater forms, and all belonging to existing species. It is hardly necessaiy 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, we have a conclusive answer to the suggestion 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 remains; 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 us 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.[2] The Seine, on the other hand, receives tributaries which drain other formations. In the valley of the Yonne we find fragments of the crystalline rocks brought from the Morvan.[3] The valley of the Oise is in this respect particularly instructive: "de Ma-
