Le Conti-;.] 



Critical Periods. 



319 



from place to place by migrations and conflicts as already explained. 

 Therefore the line of delimitation between the primary divisions 

 will not be drawn at exactly the same horizon in all places. 



I wish now to show that there have been several such critical 

 periods, or periods of very great changes affecting the whole earth, 

 and that they form a rational basis for primary divisions of the geo- 

 logical record, applicable to all parts of the earth. I begin with the 

 last; not because it is the greatest, for it is indeed in many respects 

 the least, but because it is the nearest to us and therefore the clear- 

 est, and on this account may be expected to throw light upon the 

 real nature of such periods. I shall call such periods critical peri- 

 ods or revolutions. 



1. THE GLACIAL REVOLUTION. 



The Drift or Quaternary or Glacial Period, or Ice Age, I am 

 convinced constitutes one of these critical periods and the last. I 

 have called it the Glacial Revolution. 



The cause of the glacial climate we shall not discuss. What- 

 ever it be, whether geographical or astronomical, or both, it is cer- 

 tain that it was attended with great and widespread oscillations of 

 the earth's crust by elevation and depressions in all high latitude 

 regions. The greatness of these oscillations we are only beginning 

 to understand. The North American continent, for example, was 

 certainly raised at least three thousand feet and probably much 

 more. The continent was enlarged until the shore line advanced to 

 and even beyond the submerged continental margin. In Europe 

 the elevation was equally great. It is probable also that the south- 

 ern hemisphere was similarly affected nearly if not absolutely simul- 

 taneously. From the distribution of organic forms in the southern 

 hemisphere there seems much reason to believe that the Antarctic 

 continent was elevated and enlarged until it connected with and 

 connected together the point of South America. South Africa. Aus- 

 tralia, and New Zealand. The effect of these great changes in 

 physical geography would be to produce corresponding extreme 

 changes in climate and wide migrations of organic forms, partly 

 permitted by changes in physical geography and partly enforced 

 by changes of climate. These migrations extended not only 



