Sayles — Dilemma of Paleoclimatolo gists. 469 



cene. Mountain building and vnlcanism went on for mil- 

 lions of years before the cool Pliocene and cold Pleisto- 

 cene. There is a lagging effect here which points to a 

 gradual cooling off of the earth. In this slow cooling, 

 mainly during the Pliocene but to some extent also during 

 the Miocene, the oceans cooled before the lands, as proved 

 by the migration of northern marine species southward 

 and the less rapid migration of the land flora and fauna 

 during the same time. 21 This cooling of the waters could 

 have been produced by the cooling of the lithosphere 

 under the oceans or by the cooling of the atmosphere and 

 formation of winter ice at the poles. To cool off the 

 oceans to the extent indicated would take a very long time. 

 The process was fairly steady. The effects of water 

 vapor in carrying heat can hardly be emphasized too 

 much. With the emergence of continents and the emer- 

 gence of barriers in the seas, during periods of mountain 

 building, the scope of the heating effects of water vapor 

 would be much restricted, with a cooler earth as the 

 result. However the cooling was accomplished, when the 

 glacial conditions finally did set in they came on rapidly. 

 The earth had cooled off enough for some cause, other 

 than its gradual cooling off, to act abruptly. The cooling 

 and heating, if accomplished by secular cooling, as Man- 

 son would have it, is much too slow a process to account 

 for the interglacial episodes of the Pleistocene and 

 Permo-Carboniferous. Changes in the carbon dioxide 

 content of the atmosphere is also much too slow a process 

 to explain these same interglacial episodes. 



A great many theories for ice ages have come and gone. 

 Xo theory can stand long which does not satisfactorily 

 account for interglacial episodes. Only two hypotheses 

 so far advanced would appear to the writer to have any 

 chance : changes in the heat of the sun, and periods of vul- 

 canism with great amounts of volcanic dust. If it could 

 be proved that a change in the heat of the sun would bring 

 on glaciation, when the earth had been cooled off enough 

 by continental emergence, affected somewhat by loss of 

 heat through large volcanic extrusions, then a reverse 

 change would explain an interglacial episode. That such 

 a change in the heat of the sun should come just at the 

 time when the earth was sufficiently cooled off, would 

 appear to be a coincidence. How can we understand it, 



