48 THE JOURNAL OF GEOLOGY. 



nence is more than one -half the transverse base, the lateral 

 currents will close in on the lee, and the summit current will flow 

 off into the body of the ice. This simple law is, however, subject 

 to very considerable modifications from several different sources 

 which may be grouped under (i) differences in the' friction 

 arising from basal contact, and (2) differences of internal fric- 

 tion and mobilit}'. The lateral currents will expose more surface 

 to the sides and base of the hill and the adjoining plain, and 

 will be more subject to conflicting currents, while, on the other 

 hand, being deeper currents, they will presumably be more fluent. 

 These and other qualifying conditions will go far to vitiate the 

 application of the law, but its statement may have some value 

 as representing a general conception of the phenomena. When 

 the height of the prominence becomes great relative to the total 

 thickness of the ice, the fluency of the summit current may be 

 much reduced relative to that of the central parts of the lateral 

 currents. When the prominence reaches the surface, blocks 

 dislodged from it are borne away on the surface of the glacier, 

 and constitute superglacial drift. Blocks dislodged from near 

 the summit, but below the surface of the ice, are presumablv 

 carried onward in the upper zone of the glacier ; while other 

 blocks detached at various but sufficient heights on the side of 

 the prominence are doubtless borne around into the lee and car- 

 ried forward in the same vertical plane as the summit stream, so 

 that there comes to be a vertical zone set with boulders moving 

 on from the lee side of the nunatak. 



Lofty ledges or plateaus, with vertical or undercut faces, 

 furnish similar means for the lodgment of debris within the 

 body of the ice. 



In these and doubtless in other ways it appears that there 

 came to be lodged directly within the body of the Pleistocene 

 glaciers at some considerable distances above their bases, blocks 

 derived from rock prominences that rose with sufficient steep- 

 ness above the general surface of the countr}' over which the ice 

 passed. The lodgment of debris on the lateral borders of gla- 

 ciers is neglected here because it has little or no applicability to 



