362 



HISTORICAL GEOLOGY 



and shorter but hotter summers in the northern hemisphere, 

 such as now occur in the Antarctic regions. By the latter — viz., 

 increasing eccentricity (which forms a much longer cycle) — these 

 effects, which are now small on account of the nearly circular form 

 of the earth's orbit, would become very great. At the time of great- 

 est eccentricity, the earth would be 14,000,000 miles farther off 

 from the sun in winter than in summer, the winter would be 

 twenty-two days longer and 20° colder, and the summers twenty- 

 two days shorter, 

 but much hotter 

 than now. . . . 

 Now, according to 

 Croll, the coinci- 

 dence of aphelion 

 winter, with a 

 period of greatest 

 eccentricity pro- 

 duces a glacial 

 climate." l 



As a result of 

 the astronomic re- 

 lations, Croll held 

 that the heat 

 equator, with the 

 trade winds zone, 

 must have been shifted farther away from the glaciated hemi- 

 sphere with consequent shifting of direction of warm ocean cur- 

 rents. Thus, during the Quaternary Glacial epoch, the Gulf 

 Stream must have been diverted southward by the eastern point 

 of South America. 



According to Croll's hypothesis (1) there must have been many 

 Glacial epochs during the earth's history; (2) alternations of cold 

 and warm stages (seven or eight) must have occurred during the 

 Glacial epoch ; (3) these cold and warm stages alternated between 

 northern and southern hemispheres; and (4) the Quaternary 

 Glacial epoch in the northern hemisphere began 240,000 years 

 ago, lasted 160,000 years, and declined 80,000 (or possibly 60,000) 

 years ago. 



At present we have positive evidence for five or six times of 

 1 J. Le Conte: Elements of Geology, 5th ed., pp. 613-614. 



Fig. 226 

 Diagram showing effect of Precession: A, present 

 condition; B, condition 10,500 years hence. Ec- 

 centricity much exaggerated. (After Le Conte's 

 "Geology," permission of D. Appleton and Com- 

 pany.) 



