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DRUMLIN, OSAR AND KAME FORMATION.
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it was not absolutely at the bottom of the ice, I think, in a genetic view, it is to be regarded as basal unless it was lifted so high into the body of the glacier as to be borne onward thenceforth entirely within the body of the ice and free from basal influences so that it was at length carried out to the surface as it approached the terminal edge, and was deposited as superglacial material. If the material remained approximately at the bottom of the glacier and again descended to the absolute base of the ice, it seems to me best to regard it as basal, even though it may be, for a time, completely enveloped within the ice. This seems best, because it represents the significant factor in the operation. In origin, it was basal, and, in the end, it became basal. It was only englacial by accident, temporarily.

Opposed to the agencies that tended to carry material from the absolute bottom of the ice into its basal portion to limited heights, there were several agencies that tended to bring it back to the absolute bottom. (1) The conduction of internal heat contributed slightly to this by melting away the base of the ice. The annual amount was undoubtedly very small, but the cumulative effects upon the bottom of any particular column of ice during the last five hundred miles of its journey (and this much is involved in certain aspects of the problem) was probably very appreciable and was manifestly greater in proportion to the slowness of the ice movement. (2) Basal friction undoubtedly gave rise to a much larger wastage and so lowered the embedded debris. (3) The introduction of warm waters from the surface, through the agency of crevasses, also caused wastage of the bottom, but this was obviously limited to such portions as were accessible to these waters and the effects were unequally distributed, although the positions of the streams undoubtedly changed from time to time, and this tended to spread the effects more generally over the bottom. (4) It is probable that there was a certain amount of penetration of solar rays through the ice. As the surface of a glacier is usually granular, only a minor portion of the sun's rays probably succeeded in penetrating to the transparent ice below. But such portions as reached this

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