These numbers are not high. The number given by M. Morlot as the age of the Stone period in Switzerland, leads us back to an antiquity which does not exceed that given by the Egyptian monuments; and it is impossible to avoid being struck with the differences of civilization exhibited by the two countries. Nevertheless, this fact cannot constitute a reason for doubting the results of the Swiss savant. It is well known that man during the same time has not everywhere equally advanced in civilization, and that the Esquimaux are still in the Neolithic period. But other criticisms have been brought forward against M. Morlot, the result of which is that the numbers furnished by the cone of the Tinière cannot be accepted as giving a real approximation to the date which we are seeking for. V. M. Forel, who has taken an active part in this discussion, has tried to solve the problem in an indirect way. Instead of seeking directly for the age of a prehistoric fact, he has proposed to have recourse to the rule of false position, which allows the determination either of a maximum which the numbers cannot possibly exceed, or a minimum below which they cannot fall. He has applied this plan, which is as correct as it is ingenious, to the Lake of Geneva. It is well known that the waters of the Rhone, especially during the floods caused by the melting of the snow, enter the lake in a very turbid condition, and flow on remarkably clear. The mud thus deposited evidently tends to fill up the lake, and has already silted up a part of the great depression which was filled by the ice of the Quaternary epoch. M. Forel has first determined the annual volume of the deposit. He has then calculated the volume of the present lake, basing his calculations on the soundings made by La Bèche. He has thus been enabled to calculate the time necessary for the sediment of the Rhone to fill up the entire lake. Then, admitting that the part of the original lake already filled up had a mean depth equal to that of the present lake, he has compared the surface of the alluvial deposits already formed with the surface of the lake itself. The proportion is almost one to three. These deposits have then been formed in a third of the time necessary to fill up the present lake. Now their formation commenced immediately after the retreat of the glaciers. The date thus obtained is, then, that of the modern geological epoch.