Page:Popular Science Monthly Volume 59.djvu/252

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THE fact that a very extensive and massive ice sheet covered countries of the northern hemisphere which now enjoy a mild climate is generally known and accepted, although it is little more than fifty years since Agassiz (1840-47) made the then novel suggestion to explain the occurrence of glacial deposits where no glaciers remain. It is not so generally known that the great ice age was characterized by the development of several ice sheets in succession, each of them separated from its forerunner by an interval of mild climate during which the ice retreated far toward its source, and but few realize that these intervals of mildness were longer than the time which has elapsed since the latest glaciers withdrew from New England and the northern Central States.

Since the fact of a glacial period was established, several hypotheses have been framed to account for the phenomena of climatic change. As the sun warms the earth, variations in its condition and distance were postulated. As the poles are now regions of glacial accumulation, it was thought that the earth's axis of rotation might have shifted in such a way as to bring the once glaciated regions into polar relations. Or as heights of land are often mantled in snow and ice under latitudes where lowlands are free, glaciation was connected theoretically with a general elevation of continents and mountains. There are facts to sustain most of the speculations thus suggested. Each contains a possible cause. But no one is free from serious question of its sufficiency, while there is little evidence to show that any was definitely related in time to a glacial epoch, except that one which is based on a general elevation of the land.

Professor Chamberlin, of the University of Chicago, long ago advocated a method of investigation known as the method of multiple hypotheses. It calls upon the student to lay aside a natural preference for the theory which seems plausible and to consider as sincerely that which holds out small promise of development. As an earnest student of the causes of the glacial period, he has thus considered every suggestion that might solve that enigma. The astronomical causes, the shifting of the pole, the variations in altitude of the continent, have all been passed in review,

  1. A review of Chamberlin's 'Working Hypothesis of a Cause of Glacial Epochs.'