372 G. F. Wright— Unity of the Glacial Epoch. 



to a height of 175 feet. The subsidence to the north was so 

 much greater than that at the south that the drainage of these 

 rivers was much slacker than now, but nowhere do the terraces 

 of this period on the Delaware and Susquehanna rivers -extend 

 much above 175 feet except inside the region actually covered 

 by glacial ice. How much these rivers have lowered their 

 beds since that time it is difficult to tell, but probably not a 

 great deal. 



I cannot well close this part of the discussion without refer- 

 ence to a recent attempt of Professor Winchell (Am. Geolo- 

 gist, Aug. 1892, pp. 69 to 80) to measure the length of the 

 interglacial epoch in Minnesota. Professor Winchell presents 

 strong evidence to show that the preglacial channel of the 

 Mississippi above St. Paul ran in a nearly straight line with the 

 lower valley and reached the present valley again seven or eight 

 miles north of Minneapolis. This he supposes to have been 

 tilled up during the first glacial epoch, while in the interglacial 

 epoch the channel that has long been known west of Minneap- 

 olis was eroded, to be filled on the advance of the last glacial 

 ice sheet. This amount of work he estimates would not have 

 required more than 9.750 years, which would be the length of 

 the interglacial epoch in that latitude. Though this is as 

 nothing to the length of the period as estimated by Professor 

 Chamberlin for the southern part of the glaciated area, it is 

 sufficiently long to make us ask whether the data are sufficient 

 by which he would establish the different ages of those pre- 

 glacial channels % The aspect of the unglaciated areas where 

 secular erosion is open to inspection is such as to throw much 

 doubt over surface indications in the glaciated region. I 

 should be inclined, therefore, to wait for further light before 

 committing myself unreservedly to the theory in question. 



9. The final consideration bearing on the duality of the 

 glacial period is drawn from the successive enlargement and 

 desiccation of Lakes Bonneville and Lahontan, in the arid 

 basin west of the Rocky Mountains. This coincidence is 

 certainly very striking, and the theory that there is a causal 

 connection between the phenomena of the glacial periods and 

 the enlargements of these lakes is very plausible. But in our 

 general ignorance of the causes of local climatic changes such 

 a coincidence should not go far in face of other evidence. 

 There are, for example, successive salt deposits in many regions 

 which indicate local variations of climate equal to anything 

 which can be demonstrated in the great Rocky Mountain 

 Basin, but in geological periods when we have no reason to 

 suspect glacial conditions. That the last enlargement of Lake 

 Bonneville coincided with the glacial period is pretty certain, 



