A Manual of the Antiquity of Man. J. P. Maclean
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.
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