'208 WAEKEN UPHAM, ESQ., ON CAUSES OF THE ICE AGE. 



not SO much as one degree of the average temperature of the 

 earth's chmates was ever, within geok^gic times, so received 

 from all other sources besides the sun and the earth's own 

 internal heat. Concerning the latter, also, it is well ascer- 

 tained that during at least the Mesozoic, Tertiary, and Qua- 

 ternary eras, it has affected the climatic average by no more 

 than a small fi action of a degree. 



11 Others have suggested that the sun's heat has varied, and 

 that the Ice age was a time of diminished solar radiation. 

 To tliis we must answer that during the centuries of written 

 history, and especially during the past century of critical 

 investigations in terrestrial and solar physics, no variations 

 of this kind have been discovered. Such a cause of the 

 glacial accumulations would have enveloped Alaska and 

 Siberia with ice-sheets and their drift deposits. The anoma- 

 lous geographic distribution of the drift forbids this 

 hypothesis. 



12 Among all the theories of the causes of the Glacial 

 period, the one which has attracted the most atten- 

 tion, not only of geologists, but also of pliysicists and 

 astronomers, was thought out by Dr. James Croll, and pub- 

 lished in magazine articles, during the years 1864 to 1874, 

 and is most fully stated in his work entitled, Climate and 

 Time (1875). His answers to criticisms and more full eluci- 

 dation of some portions of the theory are given in his later 

 volumes. Discussions on Climate and Cosmolofiy (188/)\ 

 and Stellar Evolution and its Uflations to Geological Time 

 (1889).* Dr. Croll's theory, which also has been very ably 

 advocated by Prof. James Geikie in The Great Ice Acje (1874 

 and 1877), and recently by Sir Robert S. Ball in Tlie Cause of 

 an Ice Age (1891), attributes the accumulation of ice-sheets to 

 recurrent astronomic cycles which bring the winters of each 

 polar hemisphere of the earth alternately into aphelion and 

 perihelion each 21,000 years during the periods of maximum 

 eccentricity of the earth's orbit. Its last period of this kind 

 was from about 240,000 to 80,000 years ago, allowing room 

 for seven or eight such cycles and alternations of glacial and 

 interglacial conditions. The supposed evidence of interglacial 

 epochs therefore gave to this theory a wide credence ; but the 

 uniqueness of the Glacial period in the long geologic record, 



* A full list of Dr. Croll's scientific papers and works is appended to an 

 interesting biographical sketch, with portrait, in Transactions of the 

 Edinburgh Geological Society, vol. vi, pp. 171-187, for Feb. 19, 1891. 



