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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 (1) differences in the friction arising from basal contact, and (2) differences of internal friction and mobility. 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 presumably 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 carried 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 steepness above the general surface of the country over which the ice passed. The lodgment of debris on the lateral borders of glaciers is neglected here because it has little or no applicability to

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