CHAPTER XX. 



THE DATE OF THE GLACIAL PERIOD. 



Two causes have combined in recent years to favor erro- 

 neous calculations concerning glacial chronology. Of these, 

 the first has been the almost unquestioned acceptance of the 

 astronomical theory subjected to examination in the preced- 

 ing chapter. If Mr. Croll's theory of the cause of the Gla- 

 cial epoch is correct, then we should no longer speak of an 

 ice age, but of a succession of such ages, whose dates could be 

 readily determined from a table showing the periods of high- 

 est eccentricity in the earth's orbit. According to this table, 

 the modem period most favorable to the production of a 

 glacial epoch began about two hundred and forty thousand 

 years ago and ended about seventy thousand years ago. The 

 whole intervening time was one of high eccentricity, when, 

 during the recurring intervals in which the winters occurred 

 at aphelion, the excess of winter over summer ranged from 

 fourteen to twenty-six days, and the intensity of the heat 

 received from the sun during those aphelion winters was ten 

 per cent less than at the present time. During the time in- 

 tervening between seventy thousand and two hundred and 

 forty thousand years ago, there occurred, therefore, according 

 to this theory, a succession of glacial and interglacial periods 

 in which geologists and archaeologists are invited to distrib- 

 ute their remarkable discoveries concerning glacial man. 

 Undue confidence in this theory has had no small influence 

 in diverting attention from the more legitimate lines of in- 

 vestigation. 



A second source of error has been an incorrect interpre- 



