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THE JOURNAL OF GEOLOGY.

were competent to traverse very considerable depths of ice without being entirely absorbed, being chiefly waves of short vibration. Those who have been beneath glaciers have observed that the amount of transmitted light is not inconsiderable. The transmitted rays of short vibration, so far as they reached the bottom, were arrested and, by transformation to rays of longer vibration, brought to bear upon the base of the glacier. The basal wastage from this source may be presumed to have increased somewhat in proportion as the ice thinned towards its margin, but this would be offset to some degree by a probable increase of surface detritus that would cut off the rays. The combined effect of these agencies would appear to have been not inconsiderable.

(5) In any vertical section of a glacier the lagging of the basal portion causes the plane of the section to lean forward, which means that each part is brought nearer to the bottom, carrying with it whatever material is enclosed. This being a general phenomenon justifies the conclusion that the tendency of the ice of the interior of a glacier is to flow obliquely forward and downward. Exceptions to this may be found when the resistance of a given portion of the base is greater than that of the portion immediately in its rear, in which case the latter may tend to flow over the former, but this will be reversed when the ratio of resistance is changed and would be, at most, but a variation of action, not a general law of action.

The combined effect of all these agencies was, if I reason correctly, to bring back to the base of the ice any material that owing to the special causes named, or to others, had been forced up into the lower parts of the ice. They tended to preserve the basal character of whatever had once become basal. And this seems to be supported by observations on existing glaciers.

These considerations have a very specific bearing upon the horizon at which drumlins, osars (eskers), and kames were formed. These all contain large quantities of local material, of basal derivation. If the view here stated is correct, these must be very strictly basal deposits, in general. There are doubtless

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