384 CAUSES OF CLIMATAL CHANGE 



(7) Differences in the distribution of land and water in con- 

 nection with the flow of oceanic currents. 



(8) Variations in the properties of the atmosphere with 

 reference to its capacity for allowing the radiation of heat. 



Something may be said in favour of all these alleged causes ; 

 but as efficient in any important degree in producing the cold 

 and warm climates of the Tertiary period, the greater number 

 of them may be dismissed as incapable of effecting such results, 

 or as altogether uncertain with reference to the fact of their 

 own occurrence. 



(1) That the earth and the sun have diminished in heat 

 during geological time seems probable ; but physical and geolo- 

 gical facts alike render it certain that this influence could have 

 produced no appreciable effect, even in the times of the 

 earliest animals and plants, and certainly not in the case of 

 Tertiary floras or faunas. 



(2) The obliquity of the ecliptic is not believed by astrono- 

 mers to have changed to any great degree, and its effect would 

 be merely a somewhat different distribution of heat in different 

 periods of the year. 



(3) Independently of astronomical objections, there is good 

 geological evidence that the poles of the earth must have been 

 nearly in their present places from the dawn of life until now. 

 From the Laurentian upward, those organic limestones which 

 mark the areas where warm and shallow equatorial water was 

 spreading over submerged continents, are so disposed as to 

 prove the permanence of the poles. In like manner all the 

 great foldings of the crust of the earth have followed lines 

 which are parts of great circles tangent to the existing polar 

 circles. So, also, from the Cambrian age the great drift of 

 sediment from the north has followed the line of the existing 

 Arctic currents from the north-east to the south-west, throwing 

 itself, for example, along the line of the Appalachian uplifts in 

 Eastern America, and against the ridge of the Cordilleras in 

 the west. 



