242 POPULAR SCIENCE MONTHLY. 



CLIMATE AND CAEBONIC ACID.* 



By bailey WILLIS, 



U. S. GEOLOGICAL SUKVEY. 



npHE fact that a very extensive and massive ice sheet covered coun- 

 -L tries 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 ex- 

 plain 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 JSTew 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 suffi- 

 ciency, 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 advo- 

 cated a method of investigation known as the method of multiple hy- 

 potheses. It calls upon the student to lay aside a natural preference for 

 the theory which seems plausible and to conside? 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 sug- 

 gestion that might solva that enigma. The astronomical causes, the 

 shifting of the pole, the variations in altitude of the continent, have all 

 been passed in review, 



•A review of Chamberlin's 'Working Hypothesis of a Cause of Glacial 

 Epochs.' 



