CHAP. XIX. SWISS LAKE-DWELLINGS. 373 



delta of the Tiniere, vegetable soils have been met with, 

 containing human bones and works of art of the Eoman, 

 the bi'onze, and the stone periods, they can only be con- 

 sidered, as yet, as being tentative, and, if a rough approxi- 

 mation to the truth has been made, it is all that can be 

 expected. (See j). 27 et seq.) The}- have led to the assign- 

 ment of 4000 and 7000 years before our time as the lowest 

 antiquity which can be ascribed to certain events and monu- 

 ments; but much collateral evidence will be required to 

 confirm these estimates, and to decide whether the numbei 

 of centuries has been under- or over-i'ated. 



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 inter- 

 mediate date will one day have to be intercalated. Of this 

 kind are those caves in the south of France, in which M. 

 Lartet has latelj^ 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 Eoman and Celtic memo- 

 rials, the whole of which has been of slow growth, in basins 

 or depressions conforming to the present contour and drain- 

 age-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 ver}^ 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 jiresent 

 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 



