THE CAUSE OF THE GLACIAL PERIOD. 307 



tively brief time, to be succeeded by a long period of rest. 

 The heat thus brought to the surface of the earth would 

 add perceptibly to that radiated from it into space in or- 

 dinary times. Something similar to this upon the sun, it 

 is thought, might produce effects perceptible upon the 

 earth, and account for alternate periods of heat and 

 cold. 



A fourth astronomical theory is that there has been a 

 shifting of the earth's axis ; that at the time of the Gla- 

 cial period the north pole, instead of being where it now 

 is, was somewhere in the region of central Greenland. 

 This attractive theory has been thought worthy of atten- 

 tion by President T. 0. Chamberlin and by Professor G. 

 C. Comstock,* but it likewise labours under a twofold 

 difficulty : First, the shifting of the poles observed (450 

 feet per year) is too slight to have produced the changes 

 within any reasonable time, and it is not likely to have 

 been continuous for a long period. But still more fatal 

 to the theory is the fact that the warm climate preceding 

 the Glacial period seems to have extended towards the 

 present north pole ujDon every side; a temperate flora 

 having been found in the fossil plants of the Tertiary beds 

 in Greenland and northern British America, as well as 

 upon Nova Zembla and Spitzbergen. 



A fifth astronomical theory, and one which has of late 

 years been received with great favour, is that so ably ad- 

 vocated by the late Dr. James Oroll and by Professor James 

 Geikie. Following the suggestions of the astronomer 

 Adhemar, these writers have attempted to show that not 

 only one Glacial epoch, but a succession of such epochs, 

 has been produced in the world by the effect of the 

 changes which are known to have taken place in the 



* See papers by these gentlemen read at the meeting of the Ameri- 

 can Association for the Advancement of Science, in Washington, in 

 August, 1891. Professor Comstock's paper appeared in the American 

 Journal of Science for January, 1893. 



