406 G. R. Wieland — Polar Climate in Time. 



From the great mass of facts, only the few just given are 

 selected, because they rivet attention to the true nature of 

 Arctic climates as we find them now, and help us to better 

 call to mind the extent of the changes which the geological 

 and paleontological record, and this is the main point, shows 

 them to have undergone. Let us note further, that the several 

 Greenland, JSTew Siberian Island, and Spitzbergen plant beds 

 show that secular diminution of heat had steadily proceeded 

 from the Mesozoic on, until, in the later Tertiary, moderately 

 sharp winters like those of the present temperate zone ruled in 

 the north polar area, which at this critical juncture presum- 

 ably took on for the iirst time in its history the land-locked 

 condition and rigorous Arctic climate such as we see to-day. 

 Then simultaneously, possibly with a brief period of low solar 

 radiation, maximum obliquity of the ecliptic, a highly eccen- 

 tric orbit, changed position of the Gulf Stream, and also may- 

 hap the emergence of boreal mountains, the glacial period 

 set in. But whether or not this was preceded after the same 

 manner by an earlier Miocene glaciation is a toj)ic I shall not 

 need to take up. 



The fundamentally important point is, that extensive Arctic 

 explorations taken together with the fossil plant record, pre- 

 clude glacial periods in the north polar area previous to Mio- 

 cene time. And as we go back in time, periods of high 

 eccentricity with changed position of the ocean currents and 

 areas of barometric pressure, tended less and less to produce 

 glaciation at either pole. They onust rather have resulted in 

 prolonged hot, or frosty, or cool, or rainy, or dry seasons. Each 

 return of high eccentricity thus witnessed, speaking compara- 

 tively of the general or average conditions of the epoch in 

 which it occurred, the most profound climatal modifications, 

 these always being greatest at the poles and diminishing toward 

 the equatorial regions where they would be scarcely felt. As 

 eccentricity is always fluctuating with a tolerable frequency of 

 maximum periods, and as these always last long enough for 

 equinoctial precession to reverse the maximum effects, the 

 polar areas have hence been throughout geological time the 

 scene of a steadily increasing and Anally stupendous shuttle of 

 climatic change. 



Nor need we, so far as the main question is concerned, carry 

 this general statement further. It does not affect, unless in its 

 favor, the force of the present arguuient, for instance, that 

 there is increasingly abundant evidence of Permian glaciation 

 in the southern hemisphere. The fact is that in the north all 

 the known evidence is against pre-Miocene glacial epochs and 

 in favor of mild climates throughout the early Tertiary, and 

 of more and more tropical climates in all of the Mesozoic and 



