Читать книгу A Manual of the Antiquity of Man - J. P. Maclean - Страница 9

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Happily for the Archæo-geologist, there is given him a point from which to start in his researches into the antiquity of his race. Without it his calculations would be very indefinite and his efforts would be shorn of much of their interest. The Glacial Epoch, that has puzzled the mind of both the geologist and the astronomer, is a guide-post where he may not only look both ways, but also estimate the length of ages and number the years of man. Nothing, then, is of more importance, in this investigation, than an understanding of the condition of the earth prior to the glacial, and the knowledge of the date and length of this epoch.

For untold ages the earth, to all appearance, had been preparing itself for the reception of man. There was an abundance of game, the forests were beautiful, the domestic animals had made their appearance, the climate was warm, the soil rich, and the coal had been formed. Everything seemed to point to a bright and glorious future for man, who had already entered upon the scene. It is true there were fierce and savage beasts to contend with. These seemed but a motive power to stir man to action and develop the resources of his mind. Should he fail for a time to overcome the wild beasts a retreat was provided in the hollow recesses of the earth. But nature felt her work was still unfinished. The earth had passed through the ordeal of fire, and withstood the devastations of water, and now her long summer must come to an end. The arctic regions had been growing colder and colder, and the change was felt in the countries to the south. The northern animals were being clothed with a hairy or woolly garment for their protection. The aspect began to be forbidding. The future prospect of man was not only gloomy, but foreboded he should perish along with the many species of animals that were gradually succumbing to the cold. Great fields of ice were slowly accumulating at both the poles, and at last, by the power of their great weight, assisted by some geographical changes, began to move toward the equator, crushing and grinding the great rocks, and either driving before them, or else destroying, every living thing in their relentless march. Slowly but surely they moved on. The mountains groaned under the enormous weight of ice. Their heads were scarred, their sides bruised, torn and cut. The icy monsters listened not to the pleadings of earth, the lowing of cattle, or the cries of man. Centuries elapsed before the sun re-asserted his power. The rays of the sun, the internal heat of the earth, and other causes, produced a change. The northern ice was broken up by the time it reached latitude 39° North America, leaving its indelible traces in the bowlders, gravel, beds of sand and clay which mark its course. In Europe this sheet of ice extended as far south as Spain and Corsica. The glaciers of the Antarctic regions extended as far as latitude 41° south.

Fauna of Europe.—Among the Fauna may be mentioned the gigantic elephants, of nearly twice the bulk of the largest individuals that now exist, which roamed in herds over England, and extended across the Siberian plains and from Behring Strait to South Carolina. Two-horned rhinoceroses wallowed in the swamps of the ancient forests. Hippopotamuses inhabited the lakes and rivers. The great cave-bear, which sometimes attained the size of a horse, and the cave-tiger, twice as large as the living tiger, preyed upon the animals of less strength than themselves. Troops of hyenas, larger than those of South America, disputed with other beasts of prey. A species of wild-cat, lynx, and leopard found retreats in the same forests. Then there was a remarkable carnivorous animal called Machairodus, about the size of a tiger, and from the shape and size of the sword-like teeth, must have been a very destructive creature. The lemming and the musk ox found a home, and the wild horse pranced about unrestrained by the hand of man. The great Irish elks swiftly moved over the ground, and must have been very numerous, as their remains occur in abundance in peat-bogs and marl-pits. Nor should it be unmentioned that there was also a species of gigantic ox nearly as large as an elephant, that subsisted on the plains. All these animals followed the retreat of the glaciers and some of them were in close proximity to the ice.

Geological Period.—The glacial epoch occurred during the geological period known as the post-tertiary. The tertiary had gradually passed away and its time had been recorded on the pages of geological history. A new epoch began to dawn. This was the epoch of ice, the birth and almost the childhood of the post-tertiary.

Probable Date.—In discussing the probable date of the glacial epoch, Sir Charles Lyell says, "The attempt to assign a chronological value to any of our geological periods except the latest, must, in the present state of science, be hopeless. Nevertheless, independently of all astronomical considerations, it must, I think, be conceded that the period required for the coming on of the greatest cold, and for its duration when most intense, and the oscillations to which it was subject, as well as the retreat of the glaciers and the 'great thaw' or disappearance of snow from many mountain-chains where the snow was once perpetual, required not tens but hundreds of thousands of years. Less time would not suffice for the changes in physical geography and organic life of which we have evidence. To a geologist, therefore, it would not appear startling that the greatest cold should be supposed to have been two hundred thousand years ago, although this date must be considered as very conjectural, and one which may be as likely to err in deficiency of time as in excess."[6]

Sir John Lubbock, in his dissent from some calculations made by Mr. Geikie on the general effect produced by rivers in excavating valleys and lowering the general level of the country, says, "As regards the higher districts, however, his data are perhaps not far wrong, and if we apply them to the valley of the Somme, where the excavation is about two hundred feet in depth, they would indicate an antiquity for the palæolithic epoch of from one hundred thousand to two hundred and forty thousand years."[7]

Dana, in his chapter on the length of geological time, says, in speaking of the time required to excavate the gorge of Niagara River, that "on both sides of the gorge near the whirlpool, and also at Goat Island, there are beds of recent lake shells … the same kinds that live in still water near the entrance to the lake, and which are not found in the rapids. The lake, therefore, spread its still waters, when these beds were formed, over the gorge above the whirlpool. A tooth of a mastodon (M. giganteus) has been found in the same beds. This locates the time in the Champlain epoch. … Six miles of the gorge have been excavated since that mastodon was alive. …

"There is a lateral valley leading from the whirlpool through the Queenstown precipice at a point a few miles west of Lewiston. This valley is filled with drift of the glacial epoch, and this blocking up of the channel may have compelled it to open a new passage.

"If, then, the falls have been receding six miles, and we can ascertain the probable rate of progress, we may approximate to the length of time it required. Hall and Lyell estimated the average rate at one foot a year—which is certainly large. Mr. Desor concluded, after his study of the falls, that it was 'more nearly three feet a century than three feet a year.' Taking the rate at one foot a year, the six miles will have required over thirty-one thousand years; if at one inch a year—which is eight and one third feet a century—three hundred and eighty thousand years."[8]

The calculation made by Dana is for the Champlain epoch. As this epoch was subsequent to the glacial, the time must be either thrown still farther back, or else allow the calculations to begin with the end of the glacial.

Probable Duration.—Lyell has attempted to form an estimate of the duration of the glacial epoch by considering "the most simple series of changes in physical geography which can possibly account for the phenomena of the glacial period," and enumerates as follows:

"First, a continental period, toward the close of which the forest of Cromer flourished; when the land was at least five hundred feet above its present level, perhaps much higher, and its extent probably greater than that given in the map, Fig. 41." (In this map the whole of the British Isles are connected with one another, and with the continent—the German Ocean and the English Channel constituting dry land).

"Secondly, a period of submergence, by which the land north of the Thames and Bristol Channel, and that of Ireland, was gradually reduced to an archipelago; and finally to such a general prevalence of sea as is seen in map, Fig. 39." (This map is intended to represent the British Isles as they appeared above water when Scotland was submerged to two thousand feet and other parts of the isles to one thousand three hundred feet.) "This was the period of submergence and of floating ice, when the Scandinavian flora, which occupied the lower grounds during the first continental period, may have obtained exclusive possession of the only lands not covered with perpetual snow.

"Thirdly, a second continental period, when the bed of the glacial sea, with its marine shells and erratic blocks, was laid dry, and when the quantity of land equalled that of the first period. … During this period there were glaciers in the higher mountains of Scotland and Wales. …

"The submergence of Wales to the extent of one thousand four hundred feet, as proved by glacial shells, would require fifty-six thousand years, at the rate of two and a half feet per century; but taking Professor Ramsay's estimate of eight hundred feet more, that depression being required for the deposition of some of the stratified drift, we must demand an additional period of thirty-two thousand years, amounting in all to eighty-eight thousand; and the same time would be required for the reëlevation of the tract to its present height. But if the land rose in the second continental period no more than six hundred above the present level … this … would have taken another twenty-six thousand years; the whole of the grand oscillation, comprising the submergence and reëmergence, having taken, in round numbers, two hundred and twenty-four thousand years for its completion; and this, even if there were no pause or stationary period, when the downward movement ceased, and before it was converted into an upward one."[9]

Lyell admits that the average rate of two and a half feet per century is a purely arbitrary and conjectural one, and there are cases where the change is even six feet a century, yet the average rate of motion, he thinks, will not exceed that above proposed. With this opinion, Lubbock believes most geologists will agree.[10]

By the estimates already given a basis is formed upon which a calculation can be made as to the time when this epoch began. At the time of the most intense cold the eccentricity of the earth's orbit was .0575; the difference in millions of miles between the greatest and least distances of the earth from the sun 10½; the number of days by which winter, occurring in aphelion was longer than the summer in perihelion 27.8; the mean temperature of the hottest summer month in the latitude of London when the summer occurs in perihelion, 113°; the mean temperature of the coldest winter month in the latitude of London when the winter occurs in aphelion, 0° 7'. Sixty thousand years later the eccentricity of the earth's orbit was but .0332; the difference of distance in millions of miles was 6; number of winter days in excess, 16.1; mean of hottest month in latitude of London, 95°, and mean of coldest month 12°. It is evident then at this time (one hundred and fifty thousand years ago) a "great thaw" had taken place and the glaciers driven back, although fifty thousand years later less intense cold set in again. If thirty thousand years be allowed for the "great thaw" from the extreme point of cold, and that extreme point to have been two hundred and ten thousand years ago, then one hundred and eighty thousand years ago the glaciers had become so broken up as to allow vegetation to spring up in many localities, and the wild beasts to partially reassert their dominion. If to this be added the time required for the duration of the glacial epoch (two hundred and twenty-four thousand years) then the time when the ice began to accumulate was four hundred and four thousand years ago. But if the tables of Mr. Croll be correct, their beginning could not have been earlier than three hundred and fifty thousand years ago, as the eccentricity of the earth's orbit varied but little from the present, and five hundred and fifty thousand years ago it was almost identical with that of the present.[11]

During the last stages of this ocean of ice it must have melted very rapidly,[12] for great rivers were formed, and the water pouring down its icy bed sought other streams, and on the bosom of the earth swept away loose sediment, depositing it along the course of rivers and in caves of the earth, covering the remains of man along with those of animals that perished during the long winter of ice.