GEOLOGICAL SEASONS. 179 



of every river and lake; and still more unequivocally along 

 the ocean's beach. 



I am of the opinion, nevertheless, that the northern 

 hemisphere has been repeatedly visited by glaciation. In 

 seeking, therefore, the explanation of the last great ice- 

 age, we must seek a theory which will explain the suc- 

 cession of ice-ages. This the older theories failed to ac- 

 complish. It was, for instance, suggested long ago as one 

 of the most obvious theoretical expedients, that perhaps 

 the poles and the equator had changed places, bringing 

 tropical climates into regions which are now frigid; or 

 that, at least, the axis of the earth had changed its posi- 

 tion, resulting in the location of the north pole somewhere 

 in the north temperate zone. But these hypotheses are 

 opposed by the stability of the movements and conditions 

 of the earth and the solar system. Indeed, changes of 

 the kind mentioned would disturb the harmony of the 

 planetary realm.* Moreover, the displacement of the pole 

 to any extent admissible by possible changes in the figure 

 of the earth, would be an insignificant cause of climatic 

 changes. Mr. G. H- Darwin has shown that to displace 

 the pole 1 46', one-twentieth of the surface of the earth 

 must be lifted ten thousand feet. All the physical changes 

 in distribution of the earth's mass which have taken place 

 since the glacial epoch could not have shifted the place 

 of the pole more than six miles. Since Silurian Time no 

 terrestrial changes have occurred which would vary the 

 place of the pole to any perceptible extent. Any change 



*Sir W. Thomson, Brit. Assoc Rep., 1876, pt. ii, p. 11; Trans. Geol. Soc. 

 Glasgoiv, iv, 313; Haughton, Proc. Roy. Soc., xxvi, 51; G. H. Darwin, Trans. 

 Roy. Soc., clxvii, pt. i; I. F. Twisden. Quar. Jour. Geol. Soc., Lond., Feb. 1878; 

 James Croll, Geological Magazine, Sept. 1878; G. B. Airy, Athenaeum, Sept. 22, 

 1869. See, also, Laplace, Systems du Monde, ed. 1824, p. 392. 



