62 H. D. MACGINITIE 



absence of insolation capable of being converted into heat energy and of 

 light of sufficient value for use in photosynthesis. The area so involved 

 will be a disc approximately 3,000 miles in diameter. Receiving no insola- 

 tion, such an area would soon dissipate any residual heat in its soil and 

 rock surfaces. There would result extended and bitter arctic cold through- 

 out the darkened area that would affect winter temperatures for consider- 

 able distances into subarctic areas. 



If we assume now that the poles have been stationary as most astron- 

 omers insist that they have, and if we assume that the continents and 

 ocean basins have been perpetuated in their present places through 

 geological time as many geologists insist that they have, we must con- 

 clude that no tropical, warm temperate, or even temperate forest flora 

 could possibly live and develop in high arctic latitudes. It would be too 

 cold on the one hand and too dark on the other hand. 



Such statements neglect entirely changes in world climates, the 

 effects of past oceanic temperatures, and the effects of oceanic and 

 atmospheric circulation. 



In order to arrive at a background for understanding later cli- 

 matic changes, we may turn now briefly to the world climate of the 

 late Cretaceous and early Tertiary. In the Upper Cretaceous the 

 continents appear to have been smaller and more occupied by 

 epicontinental seas than at any time since the Ordovician (Zeuner, 

 1945, p. 164). The continents were comparatively featureless and 

 rather uniformly of low elevation. North America was divided into 

 two subcontinents by a great inland sea extending from the Gulf of 

 Mexico to the McKenzie Delta, with an east-west breadth of about 

 12° of longitude. If we could transport ourselves back to the world 

 of the Upper Cretaceous, it would be like taking a flight to another 

 planet, so different would conditions seem. Just how different we 

 may never be able fully to know or appreciate. Tropical to sub- 

 tropical floras occupied most of the southern two-thirds of the 

 United States. When we investigate the late Cretaceous floras of the 

 far north we find an astonishing circumstance in terms of present 

 conditions. All around the North Pole, north of latitude 55°, we find 

 fossil floras dominated by temperate deciduous trees. Some of the 

 more significant localities of such floras are in western Greenland at 

 about 75° N. Lat., in Spitzbergen at about 78° N., and in the area 

 of the present Yukon Valley in Alaska at about 65° N. It has been 

 shown that these northern Cretaceous floras were zoned about 

 the North Pole just as floras are today farther south. In his work on 

 the late Cretaceous floras of the Rocky Mountain region, Dorf 



