ICE AGES 



By Sir George Simpson, K. C. B., F. R. S. 



[With one plate] 



It is now common knowledge that there have been great changes in 

 climate during past ages. The geological evidence is perfectly clear 

 that luxuriant vegetation once grew in Greenland and Spitsbergen, 

 where now the hardiest forms of vegetable life can barely exist on the 

 small areas free from permanent ice. At the other extreme, there is 

 no doubt whatever that at one period a great ice sheet covered the 

 plains of Central India and discharged icebergs into a sea covering 

 what is now the Punjab and North- West India — one of the hottest 

 parts of the earth. 



From the geological record it would appear that the climatic changes 

 of the past were in all directions, some warmer and some colder, some 

 wetter and some drier than at present; and some individual localities 

 appear to have experienced all possible climates at one time or another. 



It is the problem of meteorologists to study the extent and sequence 

 of the climatic variations in all parts of the world and to seek the cause 

 and mechanism of the changes. Very little progress has been made 

 in that direction up to the present for two main reasons. In the first 

 place, the geological evidence is very fragmentary and often doubtful; 

 and secondly, we know very little of what causes climatic variations 

 and how they are controlled. 



The geological evidence of change of climate is based on the fossil 

 relics of past vegetable and animal life and on the physical effects 

 which climatic conditions have on the surface rocks. The former is 

 very difficult to interpret. Heavy vegetation does not always mean 

 warm damp conditions, and both plants and animals may, in the 

 course of ages, change their habits so that the early representative of 

 a species which is now warmth-loving may have been developed in 

 cold surroundings. For this reason, one cannot be certain of climatic 

 conditions deduced from the remains of vegetable and animal life. 



The case is quite different when we base our climatic deductions on 

 the physical features of old land surfaces. There can be no doubt 

 when we find traces of salts left when an enclosed sea has dried up 

 that we are dealing with a dry climate in which the rainfall was less 



1 Friday Evening Discourse delivered at the Royal Institution on December 10, 1937. Reprinted by per- 

 mission of the Royal Institution from Nature, vol. 141, No. 3570, April 2, 1938. 



289 



