Such is the method by which M. Forel arrives at the number of 100,000 years. This is a maximum which is probably much exaggerated. M. Forel shows this himself very clearly. He has always taken the lowest numbers for the estimation of the increase of alluvium; he has considered on the whole year ninety days only as contributing to this increase; he has only included the Rhone in this estimation, and taken no account of other rivers, streams, etc.; he has not taken into consideration inundations, extraordinary falls of rain, landslips, etc.; he has assumed the floods of the Rhone have always resembled the present floods, while they must originally have been much more considerable, and have carried away much more material from mountain slopes but recently relieved from their covering of ice; he has said nothing of the gravel and sand which must necessarily be carried along the bed of a rapid stream like the Rhone, etc. M. Forel’s result must therefore undergo serious reduction before it approximates to the truth. Without attempting a precise statement, we can (at least) admit with almost absolute certainty that the present geological epoch commenced less than 100,000 years ago. On the other hand, M. Arcelin has sought for a solution of the same problem in the deposits of the Saône. The present river flows in a channel hollowed out in the alluvium of the Saône of Quaternary times, the banks of which have been raised by the sediment deposited during floods. The two deposits are very easily distinguished. The homogeneity of the modern alluvium indicates, moreover, a remarkably regular phenomenon. The banks of the Saône at different points form more or less abrupt hills which constitute so many natural geological sections. The erosions of the river have laid bare objects easily recognised as belonging to the Roman period, the Bronze age, and the Neolithic age. These objects are found at a constant height, showing that they are in situ. The hills of the Saône, then, constitute one of those means of estimating prehistoric chronology, which are so valuable to us. MM. Arcelin and De Ferry have attempted first to determine the age of the different layers. The numbers so obtained show a certain amount of discordance, undoubtedly due to the fact that M. de Ferry has based his calculations upon a single section, while those of M. Arcelin represent the mean taken from 33 points. The latter has, however, afterwards had recourse to the method of M. Forel, and to the rule of false position. But instead of seeking a maximum, he has endeavoured to determine a minimum. This calculation gives the following results:— Age of Roman layer 1,500 years Age of Bronze layer 2,250 years Age of Neolithic layer 3,000 years Age of Quaternary clay 6,750 years