BY H. J. JENSEN, D.SC. 61 



existence of much greater inequalities of level during these 

 periods than exist at the present day. An average high 

 altitude for the continents is, ??er se quite capable of pro- 

 ducing an ice age. 



The periods of submergence, when continents were 

 low and oceans were silting up, were periods of warm 

 temperatures ; they witnessed great organic evolution 

 and produced a rich flora in Arctic regions. Heer has 

 shown that a bounteous plant life existed in Spitzbergen, 

 and other far northern regions, in the Carboniferous, and 

 again in the Cretaceous and Eocene, that is -in the periods 

 of continental submergence or oceanic extension. No 

 such flora existed there in the Permian, or in the Triassic, 

 which were periods of continental extension. 



Professor Chamberlin, of Chicago, has ably shown 

 that these periods of Arctic warmth can easily be accounted 

 for if we assume that a reversal of deep-sea circulation 

 has repeatedly taken place. Instead of, as at present, 

 the surface currents runnino- towards the pole., and the 

 deep-sea drift moving towards the Equator, the surface 

 current-s flowed towards the Equator, and the deep-sea 

 drift was polewards. 



The drift of warm equatorial deep sea waters towards 

 the poles would ameliorate the climate there, and produce 

 conditions suitable for luxuriant vegetation. Professor 

 Chamberlin has shown, both by mathematical and 

 experimental investigation, that a slight increase in the 

 amount of salt in the sea would reverse oceanic circulation 

 in this way, and that an increase of evaporation in the 

 tropics could easily produce the required increase in 

 salinity of the ocean. Warm dense waters from equatorial 

 parts would then rise in polar latitudes, and impart tlidr 

 hoftt to the atmosphere. 



The total evaporation over the earth's surface is 

 roughly proportionate to the area covered with water, 

 and evaporation is greatest in the torrid zone. There- 

 fore, the greatest evaporation in the tropics would take 

 place when this part of the earth is almost wholly sub- 

 merged. Such a condition obtained in the Carboniferous, 

 Cretaceous, and Eocene periods, which, as mentioned above, 

 were periods of a rich Arctic flora. 



