"WARREN UPHAM, ESQ., ON CAUSES OF THE ICE AGE. 237 



northern lands was attended, in its culmination, by a less pro- 

 longed liigli uplift of the more southern drift-bearing- regions of 

 North America and Europe, and of portions of the continents 

 reaching much farther south, and that then the great ice-sheets of 

 the Glacial period were amassed. But the elevation Avas followed 

 by subsidence. Under the weight of the snow and ice, these 

 lands were finally depressed somewhat below their present alti- 

 tude, whereby, as I think, a warm temperate climate was restored,, 

 and the ice was gradually, and in a geological sense rapidly,, 

 melted away. Modera(:e fluctuations of the ice-front during its 

 general recession, like those of alpine glaciers or of the Malaspina 

 ice-sheet in Alaska, seem to me an adequate explanation of the 

 inter-glacial beds. No more sui'prise need be occasioned by the 

 occurrence of remains of warm temperate floras and faunas in 

 these beds than we must feel in seeing tropical and temperate 

 plants and animals at the foot of the Himalayas and the Alps. 

 These extensive mountain ranges, frigid and largely snow-covered, 

 doubtless exert as much influence on the climate of the contiguous 

 valleys and lowlands as could be due to the waning ice-sheets of 

 North America and Europe. Each of these ice-sheets, in its time 

 of retreat, being wasted by a warm climate at its edge, probably 

 rose to an altitude of 5,000 feet above the land within 100 or 

 200 miles back from the ice-border, which, therefore, might con- 

 siderably re-advance during any series of exceptionally cool years,, 

 with plentiful snowfall. 



Since my paper was written. Dr. George F. Becker, of the 

 United States Geological Survey, has published the results of his 

 recent mathematical investigation of the effects of the unequal 

 amounts of solar heat received by different portions of the earth's 

 surface, under varying astronomic conditions.* He cannot accept 

 the theory of the late Dr. Croll, but agrees with General Drayson 

 that increased obliquity of the ecliptic must favour snowfall and 

 ice accumulation. The greatest possible obliquity, however,. 

 Dr. Becker believes to be 24° 36', or only 1° 9' more than now. 

 His conclusions are stated as follows : — 



" I began this enquiry without the remotest idea as to what 

 conclusion would be reached. At the end of it I feel com- 



•* American Journal of Science, III, vol. xlviii, pp. 95-113, August;, 

 1894. 



