GEOLOGIC TIME. 6?$ 



zoic (including Pleistocene) respectively, the Mesozoic is given 

 a time duration of 7,240,000 years, the Cenozoic of 2,900,000 

 years, and the entire series of fossiliferous sedimentary rocks of 

 27,650,000 years. To this there is to be added- the period in 

 which all of the sediments were deposited between the basal 

 crystalline Archean complex and the base of the Paleozoic. 

 Notwithstanding the immense accumulation of mechanical sedi- 

 ments in this Algonkian time, with their great unconformities and 

 the great differentiation of life at the beginning of Paleozoic 

 time, I am not willing with our present information to assign a 

 greater time period than that of the Paleozoic — or 17,500,000 

 years. Even this seems excessive. Adding to it the time period 

 of the fossiliferous sedimentary rocks, the result is 45,150,000 

 years for post-Archean time. Of the duration of Archean or 

 pre-Algonkian time, I have no estimate based on a study of 

 Archean strata to offer. If we assume Haughton's estimate of 

 33 per cent, for the Azoic period and 67 per cent, for the sedi- 

 mentary rocks, Archean time would be represented by the period 

 of 22,250,000 years. In estimating for the Archean, Haughton 

 included a large series of strata that are now placed in the Algon- 

 kian of the Proterozoic of the United States Geological Survey ; 

 and I think that his estimate is more than one-half too large ; if 

 so, ten million years would be a fair estimate, or rather conject- 

 ure, for Archean time. 



Period. Time Duration. 



Cenozoic, including Pleistocene - - 2,900,000 years 

 Mesozoic - - - - - - - 7,240,000 " 



Paleozoic - - - - - - 17,500,000 " 



Algonkian .".....-- 17,500,000 " 



Archean io,ooo,ooo(?)'* 



It is easy to vary these results by assuming different values 

 for area and rate of denudation, the rate of deposition of carbon- 

 ate of lime, etc.; but there remains, after each attempt I have 

 made that was based on any reliable facts of thickness, extent 

 and character of strata, a result that does not pass below 

 25,000,000 to 30,000,000 years as a minimum and 60,000,000 to 



