delta of the Tinière, vegetable soils have been met with, containing human bones and works of art of the Roman, the bronze, and the stone periods, they can only be considered, as yet, as being tentative, and, if a rough approximation to the truth has been made, it is all that can be expected. (See p. 27 et seq.) They have led to the assignment of 4,000 and 7,000 years before our time as the lowest antiquity which can be ascribed to certain events and monuments; but much collateral evidence will be required to confirm these estimates, and to decide whether the number of centuries has been under or over-rated.
Between the newer or recent division of the stone period and the older division, which has been called the Post-pliocene, there was evidently a vast interval of time—a gap in the history of the past, into which many monuments of intermediate date will one day have to be intercalated. Of this kind are those caves in the south of France, in which M. Lartet has lately found bones of the reindeer, associated with works of art somewhat more advanced in style than those of St. Acheul or of Aurignac (p. 190). In the valley of the Somme, we have seen that peat exists of great thickness, containing in its upper layers Roman and Celtic memorials, the whole of which has been of slow growth, in basins or depressions conforming to the present contour and drainage levels of the country, and long posterior in date to older gravels, containing bones of the mammoth and a large number of flint implements of a very rude and antique type. Some of those gravels were accumulated in the channels of rivers which flowed at higher levels, by a hundred feet, than the present streams, and before the valley had attained its present depth and form. No intermixture has been observed in those ancient river beds of any polished Celtic weapons, or other relics of the more modern times, or of the second or 'Recent' stone period, nor any interstratified peat; and the
