278 Dr. J. Croll on the Cause of 



times *, it might indirectly affect it by increasing the mass 

 of antarctic ice, and thus increasing the force of the trade- 

 winds and the resulting northward-flowing warm currents. 

 .... And as we have seen that during the last three 

 million years the eccentricity has been almost always much 

 higher than it is now, we should expect that the quantity of 

 ice in the southern hemisphere will usually have been greater, 

 and will thus have tended to increase the force of those oceanic 

 currents which produce the mild climates of the northern 

 hemisphere " (p. 192). 



There is little doubt but that the climate of the Tertiary 

 period was greatly affected by eccentricity ; but, owing to the 

 difference in the geographical conditions of the two hemi- 

 spheres, eccentricity would exercise a much greater influence 

 on the climatic condition of the northern hemisphere when 

 the northern winter solstice was in perihelion than it would 

 do when it was in aphelion. Owing to the difference in 

 the conditions of the two hemispheres, the physical agents 

 brought into operation by a high state of eccentricity would 

 act more powerfully in impelling the equatorial waters 

 towards the Arctic regions when the winter solstice was in 

 perihelion than they would do in impelling the water towards 

 the Antarctic regions when the solstice was in aphelion. In 

 this case the northern hemisphere would be heated to a greater 

 extent when its winter solstice was in perihelion than it would 

 be cooled when the solstice was in aphelion. It is this cir- 

 cumstance which, I think, has misled geologists, and induced 

 them to conclude that because the physical agents brought 

 into operation when the winter solstice was in aphelion, 

 during a high state of eccentricity, failed to produce a well- 

 marked glacial epoch in Tertiary times, consequently the 

 climatic condition of that period was not much affected by 

 eccentricity. 



It would seem to be owing to that peculiar difference be- 

 tween the conditions of the two hemispheres that, even during 

 high eccentricity, the physical agents in operation when the 

 winter solstice was in aphelion were unable to lower the tem- 

 perature of the northern hemisphere to an extent sufficient to 

 cover high temperate and arctic regions with permanent ice ; 

 but for this very same reason these agents would be enabled 

 to raise the temperature to an extent exceptionally high when 

 the winter solstice was in perihelion. In other words, this 

 very combination of circumstances, which so much modified 



* High eccentricity might not directly modifythe mild climates, "but 

 certainly the physical agents "brought into operation by the high eccen- 

 tricity would do so. 



