QUATERNARY 



661 



gradually, in 10,500 years, brought around so as to occur when 

 the earth is farthest from the sun (aphelion) (Fig. 574 B). The com- 

 bined effect of (1) maximum eccentricity and (2) the precession of 

 the equinoxes is to make the winters 22 days longer and 20 colder, 

 and the summers 22 days shorter and hotter than now. The cold of 

 the northern hemisphere would be further intensified by the divert- 

 ing of some of the ocean currents to the south as the " heat equator" 

 moved south. In the Atlantic Ocean, if the heat equator were 



l s.p. 



Fig. 574. — Diagram illustrating the astronomical theory of glaciation. A, dia- 

 gram showing the relative positions of the earth and sun when the northern summer 

 occurs in aphelion. This is the condition now. B, diagram showing the relative 

 positions of the sun and earth when the northern summer occurs in perihelion. This 

 condition favors glaciation, since the winters are longer and colder. 



farther south, the equatorial current would be turned southward by 

 the wedge-shaped eastern coast of South America. The lowering of 

 the temperature would be further increased if elevation occurred at 

 the same time. 



Some of the objections to the theory are (1) that the various ice 

 invasions were not of equal duration as the theory requires ; (2) that 

 the duration of each glacial stage was greater than 10,500 years, in 

 some cases many times greater; (3) that during the Pleistocene 

 glaciation was greater in equatorial regions than now, where, according 

 to the theory, there should have been little change of temperature. 



3. Atmospheric Hypothesis. — An hypothesis based upon the vary- 

 ing amounts of carbon dioxide and water vapor in the atmosphere 



