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THE NATURE OF THE ENGLACIAL DRIFT OF THE MISSISSIPPI BASIN.

It is of some importance, both to the practical work of the field and the theoretical deductions of the study, to determine the nature and amount of the drift that was carried forward in the body of the ancient continental glaciers, and brought out on their terminal slopes and at length deposited at their frontal edges, and to distinguish it from that which was pushed or dragged or rolled along at the bottom of the ice.[1] It may be helpful to indulge in a speculative discussion at the outset to prepare the way for the specific evidence and the inferences to which it leads.

Whenever a prominence of rock is overridden and enveloped by a glacier of the free-moving continental type, one of two things takes place; either that part of the ice which passes over the summit of the prominence flows down its lee slope, carrying whatever debris it dislodges down to the rear base, and thence onward along the bottom of the ice, or else the currents which pass on either side of the prominence close in behind it before the corresponding current which passes over the summit reaches the point of their junction, in which case the summit reaches the point of their junction, in which case the summit current is forced to pass off more nearly horizontally into the body of the ice, carrying with it whatsoever debris it has dislodged from the summit of the prominence and embodied within its base. The law of the phenomena appears to be that whenever the height of the prominence is less than one-half the base, measured transversely to the movement of the ice, the summit current will follow down the lee slope; but whenever the height of the promi-

  1. ↑ Debris, which may be imbedded in the basal layer of the ice during some part of its transportation, but which is brought down to the bottom and subjected to basal action in the latter part of its course, and ultimately becomes a part of the basal deposit, is not here included in the englacial drift.

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