This represents a very moderate antiquity, and corresponds almost entirely with the dates of Manetho. But the minimum of M. Arcelin appears to me to be too low, and the error greater than in the case of the maximum of M. Forel. I shall only point out the most important of the causes which have led to this result. The calculations of the author are based upon the hypothesis of the equality of the floods, and of the alluvial deposit in the period between the present and the Roman period, and in times previous to that. He thus confounds the epochs when the basin of the Saône was left to Nature alone, with other epochs when the same basin was stripped of its forests, cleared and cultivated as it is at present. Now everyone knows how much more powerful the action of atmospheric agents, of rain in particular, are upon cultivated land than upon uncultivated. The upper layers, which served as the basis for the calculations of M. Arcelin, have necessarily diminished to a considerable extent the final result, since they must have been formed much more rapidly than a great part of the lower layers. I shall say, then, of the minimum of M. Arcelin what I have said of the maximum of M. Forel. It leaves us the certainty that the present geological period goes back much further than 7-8000 years. VI. What corrections ought the extreme numbers which I have just quoted to undergo in order to approximate to the truth? It is still impossible to say. But the path which should be followed in order to diminish the space which separates them is henceforth clear. The alluvium of the Saône has always appeared to me to present conditions of uncertainty which it would be difficult to overcome, and the best means of determining the age of the present period by prehistoric chronology, appears to me to be the Lake of Geneva. In order to correct the first results obtained by M. Forel, it would be necessary to take into account all the circumstances pointed out above, and several others also. It would be especially necessary, at different seasons of the year, in dry and wet weather, to gauge the smallest rivulets and ravines all round the lake, to measure the amount of mud their waters contain, and the amount of gravel and sand they carry down with it. This task is beyond the power of a single man; it would require the formation of an _Association_ for this end. The problem would be worth the trouble, and the Swiss savants, so justly proud of their beautiful lake, might easily make arrangements to obtain its solution.