894 GEOLOGY 



consin ice invasion. Using the estimates in the table of relative 

 duration above, and remembering that we are multiplying such 

 errors as there may be in the previous estimates, we reach the follow- 

 ing dates for the climaxes of the several ice invasions : 



Climax of the [Late] Wisconsin 20,000 to 80,000 years ago. 



" " " lowan 60,000 to 400,000 " 



" " Illinois 140,000 to 720,000 " 



" " " Kansan 300,000 to 1,360,000 " 



" " sub-Aftonian y to z 



Little value is to be placed on estimates of this kind, except as 

 means for developing a conception of the order of magnitude of the 

 time involved. 



Foreign 



In Europe, the succession of ice epochs and formations is not less 

 complex than in North America, though there is not complete agree- 

 ment among geologists as to the number of glacial epochs. 1 In the 

 Alps four glacial epochs are recognized. 2 These are designated 3 

 Gunz (pre-Kansan?), Mindel (Kansan?), Riss (Illinois?), and Wiirm 

 (Wisconsin?). The glacial formations of other continents have 

 not been studied in detail, in many places, but recent studies in 

 Turkestan indicate that there were several glacial epochs in the 

 Thian Shan Mountains. 4 



THE CAUSE OF THE GLACIAL PERIOD 



Many hypotheses of the cause of the glacial period have Ixvn 

 offered, but none commands universal assent. Most of them appeal 

 to a combination of agencies, but each centers on some one factor 

 which gives character to the hypothesis. They fall mainly into 

 three classes: (1) those based on elevation of the land, the /////w>- 

 metric hypotheses; (2) those based on phenomena and relations out- 

 side the earth itself, the astronomic hypotheses, and (3) those l>a>r<l 



1 Geikie, Jour. Geol., Vol. Ill, pp. 241-269. Keilback, ibid., vol. III. pp. 

 113-125. 



2 Penck, Die Alpen im Eiszeitalter. 



3 Penck, Science, Vol. XXIX, p. 359. 



4 Huntington, Explorations in Turkestan, Carnegie Institution. 



