G. F. Wright— Unity of the Glacial Epoch. 363 



closely allied to a species now growing on the hills of Lap- 

 land. The few species of mammalia have a distinctively arctic 

 facies. The Elephas primigenius, E. antiquus, the Ursus 

 spelceus, and even the Cervus elaphus and Bos primigenius, 

 are commonly associated with Reindeer, Musk-ox, and other 

 arctic animals of cold post-glacial times. Further, both the 

 trees and animals are those of our " forest bed," the last land 

 survival before the climax of the glacial period. 



"Is the return, therefore, of the retreating glacier, — sup- 

 posing the boulder gravel above the lignites of Durnten to be 

 due to direct ice action, — to be ascribed to anything more than 

 a comparatively slight temporary change of climate, like those, 

 only more marked, that now for a succession of seasons cause, 

 from time to time, a temporary advance of the glaciers ? We 

 must allow, of course, for greater differences, and possibly 

 longer intervals of time than now obtain." (Geology, vol. ii, 

 pp. 458, 459.) 



I have elsewhere called attention to the semi-arctic facies of 

 the vegetation found in our own interglacial beds. (See Ice 

 Age in N. America, pp. 482-496.) 



7. The supposed proof of the great age of the marginal 

 deposits drawn from their superior oxidation, is partly an illu- 

 sion of observers and partly a normal result of the fact that 

 the continental glacier moved over a region which was at the 

 start deeply covered with oxidized material derived from the 

 long secular disintegration which had preceded. The results 

 of this secular disintegration are abundantly evident every- 

 where south of the glaciated area, especially over the regions 

 containing granitic and gneissoid rocks. For scores and some- 

 times for hundreds of feet in depth these rocks are frequently 

 so disintegrated by the long continued percolation of water 

 charged with acids, that the mass is as loose in texture as a 

 gravel bank. (See especially " On the Formation of Bowlders 

 and the Origin of Drift Materials " by L. S. Burbank, in Proc. 

 Bost. Soc. Nat. Hist., Nov. 19, 1873 ; also a paper by Professor 

 Pumpelly, in this Journal, 1879, pp. 13-14. A valuable sum- 

 mary of discussions upon this important point is given by Mr. 

 Ralph S. Tarr in the American Geologist for July, 1892.) 

 Naturally the material first and farthest moved by the ice 

 would be this which was already well oxidized to begin with. 

 To this cause there can be little doubt is to be ascribed the 

 oxidized character of the material contained in the " fringe " 

 and of that composing the Philadelphia Red Gravel and Brick 

 Clay, which is the overwash of the ice sheet when it was at its 

 climax and when the drainage was sluggish from the differen- 

 tial northward depression which characterized that portion of 

 the period. 



