THE DATE OF THE GLACIAL PERIOD. 3(53 



tioned out to the geological periods, that we may ascer- 

 tain how much approximately can be left for the Glacial 

 epoch. 



On all hands it is agreed that the geological periods 

 decrease in length as they approach the present time. 

 According to Dana's estimates,* the " ratio for the Palaeo- 

 zoic, Mesozoic, and Cenozoic periods would be 12 : 3 : 1 " — 

 that is, Cenozoic time is but one sixteenth of the whole. 

 This embraces the whole of the Tertiary period, during 

 which placental mammals have been in existence, to- 

 gether with the post- Tertiary or Glacial period, extend- 

 ing down to the present time ; that is, the time since the 

 beginning of the Tertiary period and the existence of the 

 higher animals is considerably less than two million years, 

 even upon Mr. Wallace's basis of calculation. But if we 

 should be compelled to accept the calculations of Sir 

 William Thomson, Professor Tait, and Professor New- 

 comb, the Cenozoic period would be reduced to consider- 

 ably less than one million years. It is difficult to tell how 

 much of Cenozoic time is to be assigned to the Glacial 

 period, since there is, in fact, no sharply drawn line be- 

 tween the two periods. The climax of the Glacial period 

 represented a condition of things slowly attained by the 

 changes of level which took place during the latter part 

 of the Tertiary epoch. 



In order to estimate the degree of credibility with 

 which we may at the outset regard the theory of Mr. 

 Prestwich and others, that all the phenomena of the Gla- 

 cial period can be brought within the limits of thirty or 

 forty thousand years, it is important to fix our minds 

 upon the significance of the large numbers with which we 

 are accustomed to multiply and divide geological quanti- 

 ties.! 



* See revised edition of his Geology, p. 586. 

 f See Croll's Climate and Time, chap. xx. 



