BEFORE AND AFTER 85 



will be followed by a group of Glacial Periods alternating 

 between the northern and southern hemispheres, the time 

 elapsing between the culmination of such a period in one 

 hemisphere and in the other being about 10,500 years. 

 While one hemisphere is in a glacial period, the other will 

 be enjoying a specially mild, a " genial " period. Now, 

 according to the record of the rocks, the " genial " periods 

 were far from being those breaks in the Glacial which we 

 know as Inter-glacial periods. We have the immensely 

 long warm period of the Eocene and Oligocene, the 

 Miocene with a still warm but reduced temperature, and 

 then the gradual cooling during the Pliocene, till the 

 drop in temperature culminates in the Ice Age. More- 

 over, the duration of each glaciation during this Ice Age 

 is usually considered to have been much longer than the 

 10,000 years or so given by the Astronomical Theory. 

 Add to this that the periods of high eccentricity of the 

 Earth's orbit, though occurring at irregular intervals, are, 

 on the scale of geological time, pretty frequent ; so that 

 several of such periods would have occurred during the 

 Eocene alone. Yet the geological evidence shows un- 

 broken sub-tropical conditions in this part of the world 

 throughout the Kocene. 



