184 G. F. Wright — Continuity of the Glacial Period. 



are mostly coniferous, and such as may well grow upon the very 

 border of an ice-sheet. 



Fourthly, though supposed to be separated by sucli an enor- 

 mous interval, these two ice-invasions very nearly duplicated 

 each other ; the second extending down very near to the margin 

 of the first ; so that while the first covered an area of 13,000 

 square miles, the second covered an area of 12,000 square miles. 

 . Fifthly, Mr. McGee's interpretation of the blue till at the 

 bottom is exactly the reverse of that put forth by Mr. Leverett 

 and most observers. The upper till in this area is yellow, while 

 the lower is blue. Usually it has been thought that the yellow 

 till measured the extent of the oxidation from the surface down. 

 Of course, then, the older till should have been completely 

 oxidized, and yet throughout this part of Iowa it has the blue 

 color of the unoxidized till. Mr. McGee believes — on what 

 basis except purely theoretical it is difficult to see — that it has 

 indeed all been oxidized, but was then deoxidized through 

 the influence of the overlying forest bed. 



Altogether this report of Mr. McGee does not seem likely 

 to lend any solid support to the theory of an extremely long 

 interglacial epoch which he has so confidently espoused. 



Summary. 



In giving a brief summary of the course of events connected 

 with the Glacial period, it will come in the way to state more 

 fully than has heretofore been done how those who question 

 the long interglacial epoch can account for what has been called 

 the moraine of the second Glacial epoch, and for the river ter- 

 races which everywhere, east of the Mississippi River, head 

 near the moraine. 



So far as known, the following scheme would seem most 

 naturally to comprehend all the facts relating to the Glacial 

 period in America : — 



1st. The earlier portions of the Tertiary period were char- 

 acterized, throughout all the northern hemisphere, by low alti- 

 tude of land and a warm temperature even in close proximity 

 to the pole. 



2d. A period of slow continental elevation of the regions 

 which are now covered by Glacial drift, extending through some 

 hundreds of thousands of years, was in progress late in the 

 Pliocene epoch. During this stage of events the fiords which 

 characterize the northern portions of both Europe and America, 

 and the extensive rock gorges, like those of the upper Ohio 

 River and its tributaries, were eroded. 



3d. Contemporaneously with this continental elevation at its 

 maximum stage, and chiefly as a consequence of it, Glacial con- 

 ditions characterized all the higher, latitudes of North America 

 and western Europe. In Eastern North America the center of 



