52 IN THE GRIP OF AN ICE AGE 



elevation of the land. We saw that during the pre- 

 ceding period the earth was generally low-lying and 

 steamy. A very thick atmosphere brooded over it, 

 and, like the panes of glass of a hot-house, it kept the 

 heat down at the earth's surface. This thickness of 

 the atmosphere was partly due to the enormous 

 masses of carbon (in the shape of carbonic acid gas, 

 or carbon dioxide) in it. The great forests, which 

 absorb carbon and give out oxygen, had altered this. 

 But the dense moisture of the early atmosphere had 

 the same effect. The great rise of land now altered 

 the moisture. It swept up the cooler hill-sides and 

 was turned into rain, or "precipitated." There was 

 more running water, and less brooding moisture and 

 stagnant water. This purification of the atmosphere, 

 combined with the rise of mountain chains high up 

 above the sea level, is enough to explain an Ice Age. 



The change may have taken a hundred thousand 

 years. Indeed, it was really the last stage of a change, 

 the struggle of land and water, that we have traced 

 for some time. There was no evident upheaval. 

 What geologists mean when they call it a revolution 

 — this period they call "the Permian Revolution" — 

 is that the story of the evolution of life, taken as a 

 whole, shows at this point a more rapid and funda- 

 mental change. 



It is quite easy to see that the change of climate 

 would certainly mean a revolution for animal and 

 plant life. You m-ay bring a negro chief from Africa 



