CLIMATIC VAEIATIONS — GREGORY. 351 



Again, with the reverse case. Periods of especially active con- 

 sumption of carbonic acid were not followed by glacial epochs. As 

 Professor Chamberlin has shown, the most extensive removal of 

 carbonic acid from the atmosphere was probably during the forma- 

 tion of sheets of limestone, while coal seams contain a smaller but 

 still large amount of carbon obtained from the carbonic acid of the 

 air. The great limestone building periods fixed enormous quantities 

 of carbonic acid, which must have come from the atmosphere, be- 

 cause if obtained from the sea its fixation must have led to the trans- 

 ference of a fresh supply from the atmosphere. The greatest lime- 

 stone periods are probably the Lower Carboniferous, the Jurassic, the 

 Upper Cretaceous, and the Eocene and the Miocene. But none of 

 them was a period of active glaciation. Speaking generally, they 

 appear to have been warmer than the average. Thus in the British 

 Isles we find unusually well-developed growths of corals in the Lower 

 Carboniferous and the Jurassic. The British Eocene flora included 

 plants suggestive of a warmer climate than that of the present time, 

 while the marine faunas of the Middle Cenozoic in Europe and 

 southern Australia indicated that those seas were then warmer than 

 they are to-day. The Upper Cretaceous alone gives an}^ indications 

 of cold conditions, as shown by the probabl}^ ice-borne bowlders in 

 the English Chalk and the temperate aspect of its fauna ; but the oft- 

 stated view that Greenland then enjoyed a subtropical climate rests 

 on evidence which at least does not support the idea that the period 

 was one of universal severity. The apparent independence of the 

 times of limestone formation and glaciations is further shown by the 

 fact that the chief glacial periods — the Cambrian in Australia and 

 eastern Asia, the Upper Carboniferous or Permian of South Africa, 

 India, and Australia, and the Pleistocene in the Northern Hemi- 

 sphere — were not periods of great limestone formation. 



CHANGES IN TEMPERATURE GRADIENT OF THE ATMOSPHERE. 



The influence of changes in the composition of the atmosphere is 

 also the basis of Dickson's theory." But he traces its influence not 

 through the variations in heat absorption by the atmosphere, but 

 through the variations in the temperature gradient from the Tropics 

 to the polar regions. Dickson's paper is of value from its clear state- 

 ment of the facts showing that a development of glaciation is possible 

 with only a small change in mean temperature. 



Dickson appeals to a former difference in the temperature gra- 

 dient between the polar and equatorial regions; he attributes the 

 change in gradient either to the changes that are always in progress 



"■ H. N. Dickson, " The mean temperature of the atmosphere and the causes of 

 glacial periods," Geogr. Journ., Vol. XVIII, 1901, pp. 516-523. 



