THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE. 63 



To these may be added Prestwich's* estimate of the divi- 

 sion of the 30,000 or 40,000 years of the Glacial and Post- 

 glacial period into 15,000 to 25,000 years for the former, and 

 8,000 to 10,000 for the latter. This estimate approaches the 

 amount derived from the rate of erosion of the Niagara River 

 gorge, and the retreat of the falls of St. Anthony. f 



Mr. C. D. Walcott \ thinks the Mesozoic and Cenozoic 

 are in relation to the Paleozoic proportionately longer periods 

 than as estimated by Dana (that is, 1,3, 12 for the Cenozoic, 

 Mesozoic, and Paleozoic times respectively). 



Walcott suggests the following as probably nearer the 

 truth: Paleozoic 12, Mesozoic 5, Cenozoic (including the 

 Pleistocene) 2. He places the estimated duration of these 

 geologic divisions of time as 17,500,000 years for the Paleo- 

 zoic, 7,240,000 years for the Mesozoic, and 2,900,000 years 

 for the Cenozoic, or 27,650,000 years for the time since the 

 beginning of the Cambrian. He further estimates that the 

 Algonkian was not over 17,500,000 years, and the Archaean 

 not over 10,000,000 years more. 



Average of the Estimates of only Hypothetical Value. Ex- 

 amining the estimates from all these various sources, of the 

 length of time required to account for the deposition of all the 

 stratified rocks in which the geological record of the history 

 of organisms is preserved, we reach the conclusion that an 

 average of opinions lies somewhere between 25,000,000 and 

 75,000,000 of years from the beginning of the Cambrian to the 

 present time. Although it should be held as an extremely 

 hypothetical belief, the probabilities are considerable that the 

 time represented is within these limits rather than outside 

 them either way. 



Provisional Units of the Time-Scale Assumed to be of Equal 

 Value. But so long as the estimated value of the time-lengths 

 in geology must be considered highly hypothetical, it may be 



* " Geology," vol. n. p. 534. 



f See, further, papers by Gilbert and by Spencer on the length of time repre- 

 sented by the erosion of Niagara Falls; and, regarding the St. Anthony Falls 

 estimate, see Winchell, vol. n., "Final Report of Geology of Minnesota;" and 

 a r/sumt/ of the subject in Dana's " Manual," 4th edition, pp. 1023, etc. 



| " Geologic Time, as Indicated by the Sedimentary Rocks of North Amer- 

 ica": Proc. Am. Ass. Adv. Sci., vol. XLII. 1893, pp. 129-169. 



