520 THE ICE AGE IN NORTH AMERICA. 



SUPPLEMENTARY NOTES. 



By Warren Upham. 



The twenty-one years which have elapsed since the prep- 

 aration of this chapter have not furnished any facts mate- 

 rially to modify the views here presented. The most impor- 

 tant additional discussions of the subject are those of Messrs. 

 Chamberlin and Salisbury advocating an atmospheric the- 

 ory,* which they state as follows, when treating of the Permian 

 Glacial epoch. 



"The increased area of the land, and its increased ele- 

 vation, give increased contact between the atmosphere and 

 the rocks of the earth susceptible of carbonation and oxida- 

 tion, as already indicated. As a result, the atmosphere 

 lost carbon dioxide and oxygen at a more rapid rate than in 

 the previous period.' ' 



Here, and in other extensive developments of this theory 

 by Chamberlin in the "Journal of Geology/ ' it is definitely 

 shown that the supposed decrease of carbon dioxide, re- 

 garded as the chief cause of glaciation, is ascribed to in- 

 creased altitude of continental areas. In other words, the 

 new theory espoused by Chamberlin and Salisbury, depends 

 in the same way as my epeirogenic theory, called by them 

 the "hypsometric hypothesis," on exceptional continental 

 elevation. Each view recognizes two periods so preemi- 

 nently characterized by extensive ice-sheets, namely, the 

 Permian and the Pleistocene, that we must believe, from 

 the records of glaciation, that they each had much greater 

 land altitude, for large parts of the continents, than any 

 other period in all the vastly long ages of geologic time. 

 The great epeirogenic elevations preceding and causing both 

 the Permian and Pleistocene ice ages are well emphasized in 

 the foregoing pages; but there we had not learned how the 



* See "Geology," vol. ii, pp. 655-677; vol. iii. pp. 424-446; also 

 above p. 



