362 POPULAR SCIENCE MONTHLY. 



climate with abundant snowfall, forming an ice sheet whose dura- 

 tion extended until the land sank somewhat lower than now, lead- 

 ing to amelioration of the climate and the departure of the ice, 

 followed by re-elevation to the present level. The coincidence of 

 these great earth movements with glaciation naturally leads to 

 the conviction that they were the direct and sufficient cause of 

 the ice sheets and of their disappearance ; and this conclusion is 

 confirmed by the insufficiency and failure of the other theories 

 which have been advanced to account for the Ice age. 



The end of the Tertiary era and the subsequent Glacial period 

 were exceptionally characterized by many great oscillations of 

 continental and insular land areas. Where movements of land 

 elevation took place in high latitudes, either north or south, 

 which received abundant precipitation of moisture, ice sheets were 

 formed ; and the weight of these ice sheets seems to have been a 

 chief cause, and often probably the only cause, of the subsidence 

 of these lands and the disappearance of their ice. 



The general contemporaneousness of the Glacial period on 

 the opposite sides of the North Atlantic Ocean had been long 

 accepted as probable, but its demonstration and the identification 

 of the corresponding parts of the Ice age, having the same se- 

 quence on the two continents, were first made known less than 

 two years ago by the studies of Geikie and Chamberlin in the 

 new third edition of The Great Ice Age, and by their later papers 

 in the Journal of Geology. According to the subdivision recog- 

 nized by these authors, the time of principal accumulation of 

 marginal moraines is regarded as an epoch distinct from the pre- 

 vious portions of the Ice age; and Chamberlin has named the 

 earlier divisions of this period, when the North American ice 

 sheet reached its culmination, the Kansan and Iowan stages, 

 while the later moraine-forming time is called the Wisconsin 

 stage, from the magnificent development of the moraines in east- 

 ern Wisconsin. Between these glacial stages, which appear well 

 recognizable and synchronous in North America and Europe, 

 these authors suppose that there were prolonged interglacial 

 epochs, when the ice sheets were in large part or wholly melted 

 away. To the most important of the warm intervals, separating 

 the Kansan and Iowan stages of ice accumulation and advance, 

 the name Aftonian is given by Chamberlin, from Afton in Iowa, 

 where a thick bed of peat, formed during that time, lies between 

 deposits of glacial drift. 



Instead of this view of distinct epochs of glaciation, the Ice 

 age seems to me, while accepting the successive stages here noted, 

 to have been still essentially a single and continuous glacial pe- 

 riod, with moderate fluctuations of the ice borders during both 

 the growth and wane of the ice sheet. The marginal moraines I 



