OSBORN. REVIEW OF THE PLEISTOCENE 235 



one-third the entire Caenozoic, i. e., 1,000,000 years. If with Wallace we 

 accept Croll's theory and estimate, the last glacial advance would date 

 back to the last period of great eccentricity of the earth's orbit, namely, 

 200,000 years, but this we now consider excessive. The following figures 

 show the variations of opinion on this subject and the two opposite tend- 

 encies of greatly expanded or greatly abbreviated estimates of Pleistocene 

 time : 



Lyell, "Antiquity of Man" 186?. 800,000 years 



Upham 1893 100,000 " 



Walcott 1S93 400,000 



Sollas 1900 400.000 



Penck 1909 520,000 to 840,000 years 



The very high estimate of 840,000 years made by an eminent and 

 usually conservative authority such as Penck appears excessive unless we 

 are to expand our estimates of Tertiary time (see p. 63) to 20,000,000 

 years and of the pre-Tertiary into hundreds of millions of years. 



All the arguments for the briefer estimates of Pleistocene time have 

 recently been brought together by Wright. 21 



Antiquity of Man. — Yast interest attaches to this duration problem in 

 connection with the antiquity of man. In the calculations of Penck 22 the 

 time since the Pourth or Wurm glaciation has been used as a measure- 

 unit to calculate the length of the previous glacial and interglacial 

 periods. It is believed that since the climax of the Wurm glaciation 

 from 20,000 to 34,000 years have elapsed. Geologic, prehistoric and 

 historic events since the close of the last glaciation make this estimate 

 appear not excessive. In regard to the previous time intervals, the au- 

 thor does not pretend to give an absolute age estimate, but simply a 

 survey of the relative magnitude of the time periods with which we are 

 dealing. 



The unit of measurement is Postglacial or post-Wiirm time which 

 Penck 23 estimates at 20,000 years. On the basis of this estimate the 

 time (520,000 years) covered by the whole Glacial Epoch is relatively 

 distributed as follows : 



21 Weight, Q. Frederick : The Ice Age in North America and its Bearings upon the 

 Antiquity of Man. 8vo. Bibliotheca Sacra Co. Oberlin, 1911. 



22 Penck, A. : "Das Alter des Menschengeschlechts." Zeitschr. Ethnol., No. 3, pp. 

 390-407. 1908. 



33 Penck, A., and Bruckner. Edocabd : "Die Alpen im Eiszeitalter. Dritter Band. Die 

 Eiszeiten in den Sudalpen und im Bereich der Ostabdachung der Alpen." 8vo. Tauch- 

 nitz, Leipzig, 1909. fpp. 1153-1176, "Chronologie d. Eiszeitalters in d. Alpen." Penck.) 



