190 T. C. Chatnberlin — Diversity of the Glacial Period. 



sional discussion of the character of the drift, it is as much 

 presumed that this factor has been eliminated as, in the discus- 

 sion of an astronomical question, it is presumed that the error 

 of refraction has been eliminated. To discourse upon the 

 existence of such previously oxidized material, in a serious 

 paper written for professional glacialists, at this date, has an 

 archaic flavor. To presume that Professor Salisbury, who has 

 done some of the most critical work that has ever been done 

 upon residuary products and their contributions to the older 

 drift, overlooked this factor in the study of the Delaware 

 region, is a reflection upon the critic, rather than upon the 

 work he criticised. I have discussed with Professor Salisbury 

 upon the ground the pre-glacial factor in the sum total of oxi- 

 dation presented. To presume that a geologist cannot dis- 

 tinguish whether the aging of a pebble took place before or 

 after it became a pebble, is to suppose that this department of 

 geology is yet in a primitive condition. To assume that there 

 are no criteria for distinguishing pre-glacial oxidation from 

 post-depositional oxidation and to fail to see the applicability of 

 those criteria to the region under discussion, and make use of 

 them, is to condemn the whole work of whomsoever makes the 

 assumption ; for the evidence of post-depositional oxidation, 

 weathering, degradation and aging in its several forms, is so 

 perfectly clear and so completely demonstrative, that it should 

 not fail to impress itself upon any observer who has even a 

 moderate command of the discriminations which such a study 

 necessarily involves. If any critical student will examine the 

 terminal moraine and the drift north of it, and the gravel train 

 that leads from the moraine's outer edge down through the 

 gorge of the Delaware, on the one side, and will examine the 

 higher terraces and their flnvial deposits and the drift scat- 

 tered over the highlands, on the other, he will find clear and 

 abundant evidence that the one is relatively young and fresh, 

 and that the other is markedly old and that the two deposits 

 cannot, by any rational interpretation, be made contempora- 

 neous. 



That Professor Wright is in radical error, respecting the 

 deposits of the Delaware Valley, is evident from the self-con- 

 tradictory nature of his own interpretations. He refers the 

 slack drainage deposits along the Delaware (Philadelphia 

 Brick-clays, McGree's Columbia) to the Champlain depression.* 

 But he admits, what is incontestable, that after this epoch of 

 slack drainage deposition there was an elevation during which 

 were deposited the Trenton gravels, which he refers to a 

 "time when the ice had melted far back towards the head 



* See this Journal, Nov. No., pp. 358, 366, 370, 371 and 372. See also, " Man 

 and the Glacial Epoch," pp. 254-261 ; also, "Ice Age of N. A.," pp. 522-527. 



