Crosby.] 222 [Jan. 17, 



of the ferric oxides, in the lower portion, which shades off insen- 

 sibly into the underlying rocks, grayish and bluish tints prevail, 

 indicating that the iron is still chiefly in the ferrous state. Hence 

 the normal vertical order of colors in sedentary detritus seems to 

 be as follows, beginning at the base: 1, Bluish, grayish and 

 neutral tints due to ferrous oxide ; 2, The yellow and broWn tints 

 of the ferric hydrates ; and, 3, in warm countries, the red resulting 

 from the dehydration of the ferric hydrates. It is interesting to note 

 in this connection, that, as shown by Hitchcock and other students 

 of glacial geology, a similar contrast in colors is presented by the 

 upper and lower divisions of the till or unmodified drift. The 

 lower till, commonly regarded as the ground moraine of the ice- 

 sheet, is usually of a bluish gray color, while a yellowish or buff 

 tint predominates in the upper till, which appears to be made up 

 chiefly of materials which rested upon and in the ice and were 

 gradually deposited as it melted. 



Prof. Hitchcock has suggested that the more perfect exposure 

 of the materials of the upper till to the air during the melting of 

 the ice is sufficient to account for the more perfect oxidation of 

 the iron in the upper than in the lower till. This explanation, 

 however, requires that the drift should be mainly, if not wholly, 

 of glacial origin, and is inconsistent with the fact that, during the 

 ages since the glacial epoch, the surface of the upper till, although 

 freely exposed to atmospheric influences, shows no sensible in- 

 crease in the oxidation of the iron. While, on the other hand, we 

 can hardly doubt that the drift areas were, in preglacial times, 

 covered with a thick sheet of sedentary detritus, the upper and 

 lower portions of which must have differed in color precisely as 

 the upper and lower till do now. These considerations evidently 

 favor the view that the upper and lower divisions of the preglacial 

 detritus were not completely mixed up, or the line between them 

 entirely obliterated, by the ice sheet, but are represented by the 

 upper and lower till. It must be confessed, however, that this 

 view is not easily reconciled with our ordinary conceptions of gla- 

 cial action, and especially with the striation and scoring of the 

 underlying rocks and the presence in the upper till of numerous 

 angular blocks ; and it is offered here merely as a suggestion which 

 may assist in arriving at a correct theory of the upper and lower 

 till. 



