256 THE JOURNAL OF GEOLOGY. 



It is clear that all erratic material as it was brought to the 

 front edge of the ice appeared either on the surface or at the 

 base. There is here a sharp physical horizon of demarcation. 

 If the material that had been basal some distance back from the 

 edge was carried up to the surface, or carried up so far into the 

 body of the ice that the surface was brought down to it by abla- 

 tion near the border, it is evident that it must have become com- 

 mingled with that which had been englacial or superglacial from 

 the moment it was dislodged from its parent hills, and hence 

 this horizon of demarcation would not distinguish between the 

 two classes of material as such. The distinction would rest upon 

 mode of transportation and deposition. But if the interpreta- 

 tion of the article referred to is correct and holds good generally 

 in similar regions, the horizon becomes a plane of separation 

 between the classes of material as well, and assumes much im- 

 portance in practical glaciology. It was, however, obviously 

 not an absolute plane of demarcation, even at the border of the 

 ice, and when we attempt to apply it to sections lying farther 

 back, it needs some qualification. 



Without doubt material which was picked up by the ice along 

 its base was thrust up into it to greater or less heights. As a 

 particular instance, beds of rock which were inclined upward 

 toward the oncoming ice were obviously disposed to thrust them- 

 selves into it as they were being tilted out of their positions. 

 They appear to have rotated upon their lower edges, as upon a 

 hinge, and were probably only removed from their places after 

 they had been turned into a vertical position or perhaps some- 

 what beyond it. They were then almost wholly embedded in 

 the ice, and so, in a limited sense, they were englacial. So also 

 it is extremely probable that, in the case of undercut ledges, 

 sharp ravines, narrow gorges, and similar very abrupt inequali- 

 ties in which the surface was suddenly depressed, there was more 

 or less overriding of the basal currents of the ice and consequent 

 incorporation or overplacement of the material held in the bot- 

 tom of the overrunning portion. But notwithstanding the fact 

 that this material became englacial in a limited sense, because 



