198 Tertiary Fossils of Ca?iadaf ^c. 



Siicli changes of level must, as has been long since shown by 

 Sir Charles Lyell, modify and change climate. Every diminu» 

 tion of the land in arctic America must tend to render its climate 

 less severe. Every diminution of land in the temperate regions 

 must tend to reduce the mean temperature. Every diminution 

 of land any where must tend to diminish the extremes of annual 

 temperature ; and the condition of the southern hemisphere at pre- 

 sent shows that the disappearance of the great continental masses 

 under the water would lower the mean temperature but render 

 the climate much less extreme. Glaciers might then exist in 

 latitudes where now the summer heat would suffice to melt them? 

 as Darwin has shown that in South America glaciers extend to 

 the sea level in latitude 46° 50' ; and at the same time the ice 

 would melt more slowly and be drifted farther to the southward. 

 Any change that tended to divert the arctic currents from our 

 coasts would raise the temperature of their waters. Any change 

 that would allow the equatorial current to pursue its course through 

 to the Pacific or along the great inland valley of North America, 

 would reduce the British seas to a boreal condition. 



The boulder formation and its overlying fossiliferoiis beds 

 prove, as I have in a previous paper endeavoured to explain with 

 regard to Canada, and as has been shown by other geologists in 

 the case of other regions, that the land of the northern hem- 

 isphere underwent in the later tertiary period a great and gra- 

 dual depression and then an equally gradual elevation. Every 

 step of this process would bring its modifications of climate, and 

 when the depression had attained its maximum there probably 

 was as little land in the temperate regions of the northern hemis- 

 phere as in the southern now. This would give a low mean tem- 

 perature and an extension to the south of glaciers, more espe- 

 cially if at the same time a considerable arctic continent remained 

 above the waters, as seems to be indicated by the effects of ex- 

 treme marine glacial action on the rocks under the boulder clay* 

 These conditions, tactually indicated by the phenomena them- 

 selves, appear quite sufficient to account for the coldness of the 

 seas of the period, and the wide diffusion of the gulf stream caus- 

 ed by the subsidence of American land, or its entire diversion into 

 the Pacific basin*, would give that assimilation of the American 



* This is often excluded fi-om consideration, owing to the fact that 

 the marine fauna of the gulf of Mexico differs almost entirely from that 

 of the Pacific coast ; but the question still remains whether this diffe- 

 rence existed in the later tertiary period, or has been established in the 

 modern epoch, as a consequence of changed physical conditions. 



