BEFORE AND AFTER 83 



belong to the Glacial Period ; and as we stand in Fresh- 

 water Gate, and look at this great gap in the downs worn 

 out by the Western Yar, and think of the time when a 

 river valley passed over the tops of the High Downs and 

 Headon Hill, we receive a strong impression of the length 

 of the great Ice Age. 



Now surely the question will be asked, what caused 

 these changes of climate in the world's past history so 

 that at times a tropical vegetation spread over this land, 

 and vegetation flourished sufficient to leave beds of coal 

 within the Arctic circle, and in the Antarctic continent, 

 and at another the climate of Greenland came down to 

 England, and an ice sheet covered nearly the whole 

 country ? This still remains one of the difficult problems 

 of Geology. An explanation has been attempted by 

 Astronomical Theory, according to which the varying 

 eccentricity of the earth's orbit that is to say a slight 

 change in the elliptic orbit of the Earth, by which at times 

 it becomes less nearly circular a change which is known 

 to take place may have had the effect of producing these 

 variations of climatic conditions. The theory is very 

 alluring, for if this be the cause, we can calculate mathe- 

 matically the date and duration of the Glacial Period. 

 But, unfortunately, supposing the astronomical pheno- 

 mena to have the effect required, the course of events 

 given by the astronomical theory would be entirely dif- 

 ferent to that revealed by geological research. Geo- 

 graphical explanations have usually failed through being 

 of too local a character to explain a phenomenon which 

 affected the whole northern hemisphere, and the effects 

 of which reached at least as far south as the Equator,* 

 and are seen again in the southern hemisphere in Aus- 

 tralia, New Zealand, and South America. It is now 

 believed that great world-movements take place, due to 



* The great equatorial mountains Kilimanjaro and Ruwenzori 

 show signs of a former extension of glaciers. 



