PERMIAN GLACIATION 587 



the difference between the extremes is 3. Hence an increase of 4.2 in 

 cloudiness would apparently be accompanied by a fall of 1° F. in tem- 

 l)erature. 



The preceding discussion seems to sliow tliat at the present time the 

 net effect of an increase in cloudiness, however it may be produced, is a 

 decline of temperature. It does not seem probable that the reverse is 

 true, and that the greater cloudiness is due to a lower temperature. If 

 this were the case it would involve a seasonal change in the amount of 

 heat supplied to the earth, and this change would have to be greater over 

 the lands than over the sea. Hence we conclude that the increased body 

 of clouds actually reflects so much sunlight that the earth's surface be- 

 comes cooler in spite of the fact that the clouds hinder the radiation of 

 heat away from the earth. The amount of cooling thus occasioned is not 

 large. Even under the circumstances which we have postulated for the 

 Permian ice age, it would amount to only a degree or two. It should be 

 noted, however, that it would apparently be greater in proportion to the 

 amount of land. This would give it a maximum importance in Permian 

 times. Moreover, it would cooperate with much larger diminutions of 

 temperature arising from increased convection and evaporation. The 

 chief importance of the matter lies in the fact that it shows that at pres- 

 ent the increase of cloudiness, which is the only important positive factor 

 in raising the tempeiature of the earth's surface under our assumed 

 glacial conditions, is not able to counteract the factors of an opposite 

 character which diminish the temperature. This, then, seems to support 

 the idea that in Permian times excessive convection, evaporation, and 

 refraction may have lowered the temperature of subtropical regions to 

 such a point that the heavy precipitation was able to produce glaciers 

 which descended to low levels, 



THE PERMIAN DEkiERT IN THE NORTH 



Leaving now the equatorial and subtropical regions, let us consider the 

 probable conditions which may have existed elsewhere in Permian times 

 according to the cyclonic hypothesis. Poleward of the two subtropical 

 storm belts would lie the median zones of diminished storminess and pre- 

 cipitation. These would presumably become intensified. They would be 

 not merely arid regions, such as appear to have occupied this belt during 

 the Pleistocene glaciation, but would become vast deserts. There would 

 be especial reason for expecting this, if the lands between 40° and 70° 

 from the equator were very broadly developed, so that the oceans were 

 greatly diminished in size. During the Permian period this condition 



