This method is easily comprehended. For example, let us admit that the peat has a regular growth in the skovmoses, and suppose, in addition, that a coin, recognised as belonging to the twelfth century, has been found at a depth of 1·50 metres (4·9 feet); we shall conclude that the layer of peat has only required about 600 years for its formation. The age of a bronze hatchet found at greater depth, 8 metres (26·24 feet), will be given by the proportion lm·50 : 6 :: 8m : x. The hatchet would then be 3,200 years old, and would date from the fourteenth century before our era.
Many natural phenomena are available for calculations of this kind. Such are the alluvium of a river, the silting up of a lake, the erosion of a hill or plateau, etc. Put in order that the results of these calculations may have a real value, the phenomenon which serves as the basis, and the calculations resulting from the data must satisfy three conditions which have been very clearly stated by M. Forel.
1. The phenomenon should be perfectly constant and regular, which is never the case. At least, it ought to be possible to regard its action as giving an annual mean or constant centennial result, by means of compensations which are produced naturally.
2. When super-imposed strata are used as a means of estimation, the age of the strata serving as a term of comparison, ought to be rigorously determined; the nature of the objects compared should leave no doubt.
3. We ought to be certain that the objects found in any stratum really belong to it, that they have not been displaced by any reformation or by their mere weight. (Peat.)
Should even one only of these conditions be unfulfilled, the calculation is necessarily erroneous. Now, hitherto, we cannot be absolutely certain that the conditions laid down by M. Forel are satisfied. Nevertheless, I repeat, it is interesting to know what results have been obtained by these attempts at prehistoric chronology.
It would seem, at first sight, that the skovmoses must be useful for researches of this kind. It is not so. Steenstrup, an excellent judge of these matters, after having estimated at forty centuries the time necessary for the formation of the peat accumulated in these bogs, declares that it might be twice, or even four times as much.
In reality, the uncertainty as to the results obtained from the growth of peat, is very much greater than the Danish savant admits. In adding to the data collected by Brandt, those kindly presented to me by my colleague, M. Bésal, I find that for a period of 443 years the mean annual growth of peat is 0·032 metre (1·26 inch). But this mean is the result of numbers whose extremes are 0·065 metre (2·56 inch) and 0·0065 metre (0·26 inch). That is to say, that the means found by different observers for the annual growth of peat, vary from one to ten.
The calculations of MM. Gillieron and Troyon, resting upon the deposition of silt, which has caused the retreat of the Lakes of Bienne and Neuchâtel, have but little connection with the present subject. Both have sought to determine the age of the Lake dwellings, which belong, probably, to a much later period than the one which we are now endeavouring to determine. We may, however, notice the numbers, 6,000 years and 3,300 years, found by these observers.
The chronological results derived from the littoral accumulation of silt, of which I have just spoken, exhibit chances for error which Vogt has rightly pointed out. For some time the results have been thought more worthy of confidence which were based upon the researches made by M. Morlot upon the conical accumulation of silt deposited by the Tinière. This cone, which was cut through by the railway for a distance of 133m (436 feet), and to a depth of 7·7m (25 feet), exhibited in the midst of the mass of gravel three undisturbed soils, the highest of which contained Roman instruments and coins; the second, pottery of the Bronze age; and the third, split bones, charcoal, and different objects referable to the close of the Stone age. Fixing the commencement of the Roman period in Switzerland at the first century of our era, and the end of it at the year 563, and making some corrections which cannot be detailed here, M. Morlot has considered himself able to propose the following numbers as approximate dates:—