CHAP. IX GEOLOGICAL CLIMATES 207 



level caused by the earth's centre of gravity being shifted 

 towards the north. 



We thus see that the last glacial epoch was the climax 

 of a great process of continental development which had 

 been going on throughout long geological ages ; and that it 

 was the direct consequence of the north temperate and 

 polar land having attained a great extension and a con- 

 siderable altitude just at the time when a phase of very 

 high excentricity was coming on. Throughout earlier 

 Tertiary and Secondary times an equally high excentricity 

 often occurred, but it never produced a glacial epoch, be- 

 cause the north temperate and polar areas had less high 

 land, and were more freely open to the influx of warm 

 oceanic currents. But wherever great plateaux with lofty 

 mountains occurred in the temperate zone a considerable 

 local glaciation might be produced, which would be 

 specially intense during periods of high excentricity ; and 

 it is to such causes we must impute the indications of ice- 

 action in the vicinity of the Alps during the Tertiary 

 period. The Permian glaciation appears to have been 

 more extensive, and it is quite possible that at this remote 

 epoch a sufficient mass of high land existed in our area 

 and northwards towards the pole, to have brought on a 

 true glacial period comparable with that which has so 

 recently passed away. 



Estimate of the com^parative effects of Geograpliical and 

 Astronomical Causes in producing Changes of Climate. — It 

 appears then, that while geographical and physical causes 

 alone, by their influence on ocean currents, have been the 

 main agents in producing the mild climates which for such 

 long periods prevailed in the Arctic regions, the con- 

 currence of astronomical causes — high excentricity with 

 winter in aphelion — was necessary to the production of the 

 great glacial epoch. If we reject this latter agency, we 

 shall be obliged to imagine a concurrence of geographical 

 changes at a very recent period of which we have no 

 evidence. We must suppose, for example, that a large 

 part of the British Isles— Scotland, Ireland, and Wales at 

 all events — were simultaneously elevated so as to bring 

 extensive areas above the line of perpetual snow ; that 



