ESTIMATES OF THE DURATION OF THE QUATERNARY ERA. 99 



portions of the lake area were uplifted 400 feet or more, and the greater 

 part of the elevation took place during the first quarter or third of the 

 lake's existence — that is, at a probable rate of about one foot yearly. 

 Latest of all, the regions having postglacial marine beds above the till, in- 

 cluding the basin of Hudson bay, the Ottawa and Saint Lawrence valleys, 

 and the basin of lake Champlain, were raised from their partial marine 

 submergence, so that these recent beds containing sea shells are 300 to 

 500 feet above the sea level. A wave of mainly permanent uplift fol- 

 lowed and closely attended the recession of the ice-sheet from the south- 

 ern boundary of the drift to the central district, where the ice was thickest 

 and its remnants were doubtless latest melted aAva}^ 



Estimates of the Duration of the Quaternary Era. 



The observations of the volume of the lake Agassiz beaches, as before 

 stated, show for that lake a geologically very short existence, of about 

 1,000 years. All the conspicuous, far extended moraines, belonging only 

 to the area of the later drift, were probably formed, like those traversing 

 the lake Agassiz region, in a short time, apparently no more than twice 

 that of the glacial lake. A similar or perhaps longer time seems to me 

 requisite for the preceding reluctant and wavering recession .of the ice, 

 under the warm climate In'ought by the depression of the land, from its 

 outermost limit across the area of the early drift to the first of these 

 moraines. I am therefore led by this review to concur with Prestwich 

 in his opinion expressed as follows, concerning the duration of the Ice 

 age: 



For the reasons before given, I think it possible that the Glacial epoch — that is 

 to say, the epoch of extreme cold— may not have lasted longer than from 15,000 to 

 25,000 years, and I would, for the same reasons, limit the time of . . . the 

 melting away of the ice-sheet to from 8,000 to 10,000 years or less.* 



Arranged in chronologic order, we have derived for the three parts of 

 the Quaternary era, as here defined, the following estimates of their 

 duration : the time of preglacial epeirogenic elevation, with the deposi- 

 tion and erosion of the Lafiiyette beds, some 60,000 to 120,000 years ; the 

 Glacial period, regarded as continuous, without interglacial epochs, 

 attending the culmination of the uplift, but terminating after the sub- 

 sidence of the glaciated region, 20,000 to 30,000 years, and the Post- 

 glacial or Recent period, extending to the present time, 6,000 to 10,000 

 years. In total, the Quaternary era in North America, therefore, has 

 comprised probably about 100,000 or 150,000 years, its latest third or 

 fourth part being the Ice age and subsequent time. The Tertiary era 



* Geology, vol. ii, 1888, p. 5o4. 



