We may apply these modifying considerations to the case given by Lyell (Antiquity of Man, pp. 27, 28) of the drift deposits near the Lake of Geneva. Here are five inches in thickness deposited since the Roman period (known by its enclosed memorials) which we safely put at 1800 years. Next below is a strata of six inches depth, marked by bronze implements, which he estimates to reach back from the present time, 3000 to 4000 years. Similarly, the next strata (seven inches) indicated as the Stone age, he counts at 5000 to 7000 years old. But if the depositions were much more rapid in the early than in the later ages of our world, these estimates for the ages of bronze and of stone must be materially shortened, and may reasonably be brought within the historic period of man. (2.) The time required for the formation of _peat beds_ has been usually estimated upon its observed growth and accumulation at the present day. Yet in the case of peat-growth as in the case of drift-deposits, it is at least possible and would seem highly probable that its growth and deposition were much more rapid during the earlier ages of our race than at present. The virgin soil was richer; the climatic influences may have been more propitious. It should be considered also here (as in the case of drift) that the remains of man and his implements, instead of resting invariably upon the surface of the peat, may by various means have gone down much below the surface. The time of man’s presence, therefore, as measured by the time estimated to be necessary for the deposit of the peat found above him, may be quite overestimated. The peat beds of Denmark are put by Lyell (Antiquity of Man, p. 17) at a minimum of 4000 years. In the valley of the Somme (France) they are found 30 feet deep; and in its upper strata there are Romish and Celtic memorials, showing that its depositions continued a considerable time after the historic age of Rome. (3.) The time required for the succession of forest growths since the appearance of man. Geologists find in Denmark, earliest, a growth of Scotch fir; next, of oak; last, coming down to the present, of the beech. The age of civilization known as the Stone age synchronizes nearly with the fir; the Bronze age with the oak; the historic period with iron implements answers to the beech.[17] Now the problem is――How much time is required for one species of forest growths to run its course and become supplanted by another?――――Obviously this problem must depend not on time alone, but on climatic changes. Moreover, one kind of trees may require less time than another to exhaust the soil of the elements specially congenial to its health, vigor and stability. I do not see that any reliable measure of time can be found for estimating the life-period of different species of forest growths.