Page:Popular Science Monthly Volume 68.djvu/326

322 upon temperature, which itself would be dependent upon local conditions, 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 temperature, 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 combination of any or all of these causes, is the great problem awaiting solution, if solution is possible on other than a theoretical basis. Chamberlin, 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 hypothesis 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.