PRESIDENTIAL ADDKESS. 537 



perhaps four, warm interglacial epochs.* Not having yet found an opportunity to 

 make myself sufficiently acquainted with the evidence, I may not fully recognise 

 its importance ; but it appears to me that the factors governing the glaciation of 

 this Alpine region may have been very different from those that controlled 

 the lowland glaciation. And although it is certain that the great extension of 

 the Alpine glaciers was due to the same glacial conditions that gave rise to the 

 lowland ice-sheets of Northern Europe, I do not regard it as a necessary conse- 

 quence that advances and retreats of the ice should occur simultaneously in both 

 regions. Variation in the relative amount of snowfall over the glaciated areas 

 during the course of the Glacial Period, for which there is much evidence, would 

 be likely to produce great effects in the high-lying reservoirs of the Alps ; and at 

 the latitude of this region we should expect rapid recession of the low-level 

 glaciers in response to diminished supply. To distinguish between the effects of 

 oscillations in precipitation and of oscillations in temperature under such con- 

 ditions must be peculiarly difficult. 



North America. — In North America, where both the drifts and their literature 

 attain gigantic proportions, the state of opinion is closely analogous to that among 

 ourselves. It is agreed by all that during the Glacial Period there were very 

 extensive oscillations in the borders of the ice-sheets ; and by some geologists 

 some of the stages of recession are supposed to represent mild epochs of actual 

 ' deglaciation ' ; while others, fewer in number, among whom Mr. Warren Upham 

 and Dr. G. F. Wright have been the most active, regard these stages as of minor 

 consequence, and advocate the essential unity of the glaciation. And between the two 

 extremes stand the great majority of the workers in American glacial geology, who 

 refrain from expressing positive opinions, but mostly lean toward the idea of at least 

 one great interruption in the glaciation. Some of the suggested schemes of classiti- 

 cation * are fully as elaborate and complex as that proposed for Europe, but it seems 

 to be recognised that tliese are only of local value. Professor T. C. Ghamberlin 

 and his fellow-workers in the North-Central States have, however, adopted a 

 sequence based on the successive advance of different ice-lobes, which is believed 

 to be of wider application : and Professor Ghamberlin has tentatively suggested 

 that some of these divisions may have their counterpart in the European scheme, 

 but is careful to show that the correlation must at present remain entirely hypo- 

 thetical,' especially as the proposed American grouping may itself require 

 modification. 



It is well established that the American ice-sheete, like their European 

 equivalents, radiated from several distinct centres that attained their maximum 

 inffuence consecutively, and not simultaneouslj-. Of these the ' Laurentide ' and 

 the * Keewatin ' sheets had their radiants over comparatively low ground east and 

 west of Hudson Bay, while the ' Cordilleran ' sheet spread outward from the 

 Western Mountains. In his general discussion of the glacial phenomena of 

 North- Western Canada, Mr. J. B. Tyrrell * concludes that the Cordilleran sheet 

 had reached its greatest extent and had retired before the boulder-clay of the 

 Keewatin sheet was laid down ; and that the Keewatin sheet, in turn, had gone 

 south to its farthest limit, and had retired for many hundreds of miles — more than 

 half-way to its gathering ground — before the Laurentide sheet had reached its 

 greatest extension. 



If these conclusions be accepted, they must imply that at least in some cases 



' ' Die Alpen im Eiszeitalter.' Leipzig (1901-5), not yetcomplete ; for convenient 

 summary see ' Glazialexknrsion in die Ostalpen.' No. 12 of Guides to Excursions 

 of the Geological Co7igress, Vienna, 1903. 



' e.g., ' The Diversity of the Glacial Period in Long Island,' by A. C. Veatch, 

 Journ. Geol., vol. xi. (1903), pp. 762-776. 



* 'Classification of American Glacial Deposits.' Journ. Geol., vol. iii. (1895), 

 pp. 270-277, and in J. Geikie's Great Ice Age, 3rd ed., chap. xli. See also Ghamberlin 

 and Salisbury's recent text-book, Geology : Ea/rth History, yo\. iii. chap, xix (London, 

 1906). 



* 'The Glaciation of North-Central Canada.' Journ. Geol, vol. vi. (1898), pp. 

 J47-16} ; and ' The Qenpsis of Lake Agassiz,' ibid., vol. iv, (1896), pp. 8U-815. 



