EXGLACIAL DRIFT OF THE MISSISSIPPI BASIN. 49 



the phenomena of the upper Mississippi basin. It is also doubt- 

 ful whether any prominences protruded through the ice except 

 near the thin edge, when advancing and retreating, and these 

 are too inconsiderable to merit attention. 



It is obvious, upon consideration, that blocks detached from 

 summits or from the sharp angles of out -jutting ledges or 

 plateaus might suffer some glacial abrasion in the process of 

 their dislodgment and transposition along the crest or projecting 

 angle, but that in general such abrasion would be small, and, in 

 most cases, nearly or quite absent. The debris so incorpo- 

 rated in the body of the ice would be, for the most part, angu- 

 lar, and, as it was brought forward in the ice, it would probably 

 suffer very little abrasion. If it continued to move forward in 

 the plane in which it started, descending only so much as the 

 bottom wastage of the ice required, it would be brought out to 

 the terminal slope of the ice sheet by virtue of the melting 

 away of the ice above, and thence it would be carried on down 

 the terminal slope as superglacial debris, and dropped at the 

 frontal edge. If this be the true and full history, there would 

 be no commingling of this englacial matter with the subglacial 

 debris. It is evident that the englacial matter brought forward 

 from the crest of one prominence would be intermingled w^ith 

 that brought forward from other prominences lying in a line 

 with it, or lying so near it that the lateral spreading of the 

 debris would lead to commingling. It is also clear that varia- 

 tions in the direction of currents would tend to the same result, 

 so that englacial matter from different prominences of the same 

 general region might be commingled. So also englacial material, 

 by crevassing and by the descent of streams from the surface to 

 the base, would be carried down to the bottom and mingled 

 with the subglacial debris. So also blocks broken away from 

 the base of the prominence which yielded the englacial erratics 

 might be moved forward along the bottom parallel with the 

 englacial material above, and lodged at any point along the line. 

 It is therefore to be expected that the basal deposits will con- 

 tain the same rock species as the englacial, but if there be no 



