84 GEOLOGY OF THE ISLE OF WIGHT 



the contraction by cooling of the Earth's interior, and 

 the adjustment of the crust to the shrinkage.* Possibly 

 some explanation might be found in these world-wide 

 movements ; but their effect seems to last through too 

 long periods of time to suit our Ice Ages. Again, while 

 the geographical distribution of animals and plants in 

 the present and past seems to imply very great changes 

 in the land masses and oceanic areas, f these changes 

 appear to bear no relation to glacial epochs. The cause 

 of the Ice Ages remains at present an unsolved problem. 

 More than one Ice Age has occurred during the long 

 geological history. The marks of such a period are found 

 in Archaean rocks, in the Cambrian, when glaciers flowed 

 down to the sea level in China and South Australia within 

 a few degrees of the tropics, and above all in early Per- 

 mian times. The Dwyka conglomerate of the Karroo 

 formation of South Africa (deposits of Permo-Carboni- 

 ferous age) show evidence of extensive glaciation ; de- 

 posits of the same age in Northern and Central India, 

 even within the tropics, a glacial series of great thickness 

 in Australia, and deposits in Brazil, appear to show a 

 glaciation greater than that of the recent glacial period. 

 Yet these epochs formed only episodes in the great 

 geological eras. On the whole the climate throughout 

 geological time would seem to have been warmer than at 

 the present day. It may, perhaps, be doubted whether 

 the earth has yet recovered what we may call its normal 

 temperature since the Glacial Epoch. 



Note on Astronomical Theory. If the Ice Age be due 

 to the increased eccentricity of the Earth's orbit, the 

 theory shows that a long duration of normal temperature 



* For an account of such movements, see Prof. Gregory's Making 

 of the Earth in the Home University Library. 



t See The Wanderings of Animals. By H. Gadow, F.R.S., Cam- 

 bridge Manuals. 



