PHYSICAL AND CLIMATAL CONDITIONS. I47 



years ago, and this must have been at the close of the 

 glacial period, whatever views we may take of the nature 

 of that period. 



The estimate derived from Niagara is confirmed by the 

 ingenious and careful calculations of Winchell* respecting 

 the recession of the falls of St. Anthony, on the Missis- 

 sippi, and by those of Andrews,-|- on the lake margins of 

 lake Michigan. The former gives a period of between 

 6,276 and 12,103 years, or an average of 8,859 years. 

 The latter gives a period of from 5,290 to 7,490 years. 

 Humphreys and Abbott deduce similar figures from the 

 rate of deposit of the delta of the Mississippi. Prestwich 

 has deduced similar conclusions for England from his 

 careful and detailed observations of the later Pleistocene 

 deposits in that country.^ His estimate of the final 

 disappearance of the ice-age is from 8,000 to 10,000 

 years, and no English geologist is of greater experience 

 and authority in Pleistocene Geology. 



It may be objected that all these data are very uncer- 

 tain. This is true, but since these and a vast number of 

 facts of similar character which might be cited from 

 different parts of the world all point in one direction, their 

 cumulative evidence becomes very strong: on the one 

 hand in proof that the close of the glacial period is very 

 recent, and on the other that it must have been caused by 

 telluric changes, and these, geologically speaking, not of 

 very great magnitude. 



With reference to the connection of man with the 

 Pleistocene ice age, the present tendency of the geological 

 facts is toward the conclusion that man had his origin in 



* Journal Geol. Society, Nov., 1878. 

 t Trans. Chicago Academy, Vol. 11. 

 X Journal of Geol. Soc. of London, Aug., 1887. 



