322 POPULAR SCIENCE MONTHLY 



upon temperature, which itself would be dependent upon local condi- 

 tions, which again might, or might not, be due to elevation of land 

 surfaces. His idea, in brief, was that while during the glacial epoch 

 there might be over the entire globe a period of sufficient warmth to 

 produce the desired evaporation, the precipitation would fall as rain 

 or snow, according to the local uplift or depression. That the glaciers 

 are now retreating in nearly every instance, he regarded as due, not so 

 much to a change in climate, at least not to a gradual increase of tem- 

 perature, but rather to a gradual decrease in the amount of annual 

 precipitation. 



In this connection, it may be mentioned that Whitney considered 

 the movement of glacial ice due to water: 



Glacier ice is not simply ice, but a mixture of ice and water, and it is 

 to the presence of the latter that the whole mass owes its flexibility. The 

 larger the amount of water, other things being equal, the more easily the 

 glacial mass moves. When the water increases so as to get the upper hand, 

 the ice gives way with a rush and becomes an avalanche. . . . The extreme 

 variation of the rate of motion of different glaciers coming down from the 

 inland ice of Greenland is due to the different amounts of water which they 

 have imbibed. 



More recent observations than those quoted are familiar, and we 

 may well stop here. That, at a period geologically not very remote, 

 a vast sheet of ice and snow, with all the attributes of a modern glacier, 

 or series of glaciers, covered the northeastern United States and eastern 

 Canada, that this sheet advanced, retreated, and again advanced, and 

 finally utterly disappeared, is the commonly, though not universally, 

 accepted view. The causes which led up to this condition are still 

 problematical. Whether due to cold from increased elevation, as 

 taught by Dana, to astronomical causes, as taught by Croll, or merely 

 to an increase in precipitation, as argued by Whitney, or to a combina- 

 tion of any or all of these causes, is the great problem awaiting solu- 

 tion, if solution is possible on other than a theoretical basis. Cham- 

 berlin, the Dawsons, Gilbert, Hall, the Hitchcocks, Lewis, Mather, 

 Newberry, Salisbury, Upham, Winchell, Wright and a score of others 

 have made us acquainted with the physical characteristics of drift 

 deposits and their geographic distribution, but the first-named alone, 

 among Americans, has put forward a satisfactory working hypothesis 

 as to the cause of glacial motion. 



Leaving out of consideration Peter Dobson, whose views were not 

 pushed to their legitimate conclusions, the world at large must credit 

 Louis Agassiz, born in Switzerland, but an American by adoption, with 

 being the great promoter and, perhaps, originator of the glacial hypoth- 

 esis as it exists to-day. His method of procedure, it is interesting to 

 note, consisted in applying what one of our prominent geologists has 

 slightingly referred to as the principle of prolonging the harmless and 

 undestructive rate of geological change of to-day backwards into the 

 deep past. 



