I'HYSICAr. AND CLIMATAL COXDITIONS. 147 



yeiir.s aifo, and this iiiusl havo Itccii at iho dose of the 

 yhicial period, whatever views wcf may take of the nature 

 of that period. 



The estimate derived from Niaj^'ara is coiuiiiued by the 

 inj^enious and careful calcuhitions of AViiichell* respecting 

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

 si])jii, ami by those of Andrews,-f- on tlie lake niart^ins of 

 lake Miclii<,Mn. The former gives a period of between 

 (),270 and 12,10.'^) years, or an average of 8,859 years. 

 The latter gives a ]>eriod of from r),290 to 7,490 years. 

 Humphreys and Abbott deduce similar figures from the 

 rate of deposit of the delta of the Mississippi, rrestwich 

 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 (Jeology. 



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

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

 facts of similar character which might l)e cited from 

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

 cumulative evidence l)ecomes very strong : on the one 

 band in pvxif that the close of the glacial ])eriod is very 

 recent, and on the other tliat 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. II. 

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



