THE GREAT ICE AGE.
yr() THE EDITOR OF THE "SPECTATOR?]
Ste,—Your correspondent, Mr. Joseph J. Murphy, has always contended that the Ice Age would not be produced by the Northern hemisphere having its winter solstice when the earth is in aphelion, at a period of maximum excentricity. He looks to the cold summer at that period, rather than to the extremity of the winter, as the cause of the glacial epoch. The condition of the Southern hemisphere would favour that view, rather than the one adopted by Professor Geikie, for south of the Equator, with a higher mean winter temperature and a lower summer temperature, the perpetual snow-line comes nearer to the sea-level than it does north of the Equator. Nevertheless, if the maximum excentricity of the earth's orbit would occasion, as stated, one-fifth less heat in the summer solstice, when the earth is in aphelion, it must not be forgotten that at that period the earth would obtain one-fifth less cold in the winter solstice when in perihelion ; besides which, the summer would be longer and the winter shorter. Could, then, a winter of less severity than at present have given us glaciers that moved across Scotland, England, and Ireland, and that break off in icebergs in the Atlantic Ocean ?
Mr. Croll's hypothesis, adopted by Professor Geikie, of the increased excentricity of the earth's orbit being the cause of the Ice Age, whether with or without the changes produced by the precession of the equinoxes, will have, 1 believe, sooner or later to be given up as untenable ; for with a northern winter solstice, and the earth in aphelion, if the winter cold was excessive, so in pro- portion would be the summer heat when in perihelion ; and if, on the other band, the northern hemisphere had the summer solstice in aphelion, though the cool summer would not melt the ice so rapidly, there would be in temperate zones very little ice to melt. If it should be, as I suggest, that the greater excentricity of the earth's orbit was not the cause of the glacial period, then we have no data by which to determine when that "Great Ice Age" took place, and any relation that man may be supposed to have had to that ice age proves nothing respecting his antiquity.—I am, Sir, &c.,
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