(3.) The time required for the succession of forest growths since his first appearance.
(4.) The age of the animal races, extinct or living, whose remains are found associated with his.
(5.) We have next and last another source of testimony which is mainly free from the uncertainties of estimate, viz. the question of commercial relations between the barbarous stone-age, bronze-age, or iron-age tribes, and the civilized nations of the early historic ages.
The estimates on these several points demand distinct consideration.
(1.) The estimate of the time required for the alluvial deposits along the banks of rivers, has been extremely various.Lyell, having visited the delta of the Mississippi river in person, estimated its time-period of accumulation at one hundred thousand years.[14] But a careful examination made by gentlemen of the Coast Survey and other United States officers, reduces this time-period to four thousand and four hundred years.[15] Again, Mr. Lyell estimates that 220,000 years are necessary to account for changes now going on upon the coast of Sweden. Later geologists reduce the time to one-tenth of that estimate. A piece of pottery was discovered deeply buried under the deposits at the mouth of the Nile. It was confidently asserted that the deposits could not have been made during the historic period,until it was proved that the article in question was of Roman manufacture.[16] Such diversities suffice to show at least that somebody has blundered. Some of these high estimates are gratuitously extravagant. All estimates from the drift deposits, bearing on the antiquity of man, ought in reason to be made with careful reference to these two modifying considerations:
(a.) That drift deposits may have been, and with theutmost probability were, much more rapid in the earlier ages than at present. At the close of the glacial and ice period vast masses of loose matter were ready to be swept rapidly as drift by river freshets. Any farmer may have an illustration of this if he will plow his side-hill field, running his furrows up and down the hill. He will find that the first powerful shower will bring down far more drift than the fortieth. It would be very short-sighted in him to take the drift of the tenth year after the said plowing for his rate of annual deposition and estimate the whole period from this data. But on this mistaken principle some geologists have made their time estimates for the drift simply monstrous.
(b.) Human remains and tools may in many ways get far below the surface of the drift. They may have been buried under it after its deposition. While the drift lay under water, (soft and pliable therefore,) flints, arrow-heads, knives, or human bones, may have sunk in the mire.——These and similar considerations may demand large abatement from the time-estimates built upon the amount of drift found above the remains of man.
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 duringthe 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.