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ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 8 



than the evaporation. But of all evidences of climate those of ice 

 are the most unmistakable. At the same time one has to be careful, 

 for today the presence of ice does not always mean a cold climate. 

 In mountain regions glaciers descend the valleys into climates which 

 are far from arctic, and in New Zealand especially, ice action is taking 

 place in valleys which bear an almost subtropical vegetation. There 

 have been periods in the past when ice formation was much more 

 active than at present and of such an extent that only a radical 

 change in climate can have been responsible. 



The evidence of the presence of ice during these periods in places 

 where ice is quite impossible with the present climate is so clear that 

 these Ice Ages, as they are called, are the most appropriate subjects 

 for the study of climatic change. If we can find a cause for an Ice 



Figure 1. — Pleistocene glaciation: Ice on land shown by white, ice on sea shown by light stipple. 



Age we shall have made a long step forward toward explaining all 

 changes of climate. 



PRE CAMBRIAN ICE AGE 



In the early days of geology, when evidence of tropical vegetation 

 was found in regions which are now far from the Tropics, it was sup- 

 posed that the high temperature required by these plants was due 

 to the earth still remaining hot from its earlier molten state. The 

 discovery, however, of clear evidence of ice in pre-Cambrian times, 

 that is, at a time far anterior to the appearance of vegetation, shows 

 that the earth had by then already cooled to its present temperatures. 

 Evidence of pre-Cambrian ice has been found in North America, 

 Europe, China, South Africa, India, and Australia. Some geologists 

 have concluded from this widespread evidence of ice that the pre- 

 Cambrian Ice Age was extremely severe. So little is known, however, 



