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 would attack the projecting parts with greater power and effect than the minor hollows, and so preserve an approximate uniformity, or an average angle of moderate inclination. But when a monstrous glacier descended into a comparative plain, or into a low flat valley, the case was different. There, to use homely phrases, the ice had time to select soft places for excavation, and there, if from the confluence of large glaciers, or for other reasons, the downward pressure of the ice was of extra amount, the excavating effect, I contend, must have been unusually great in special areas, and have resulted in the formation of rock-bound hollows."

He accounted for the deep parts of the lakes by supposing that—

§ 10. "The grinding action lasted after a glacier had retired above the position of the present lake-barrier, so that the waste of the rocky floor being long continued, by degrees the glacier wore out a depression deeper and deeper, till, on its final retirement, the space once occupied by ice became filled with the water drainage of the valley."

The shallowness at their mouths was thus explained:—

§ 11. As the glaciers "progressed and melted, the ice must have been thinner, and must have exercised less erosive power than where it was thick, whence the gradual slope of the bottom of these lakes towards their outflows."

§ 12. "Therefore I have been forced to the conclusion, from a critical examination of many of the lakes in and around the Alps, that their basins were scooped out by the great glaciers of the glacial period."

The astonishment which Professor Eamsay's theory created had not subsided when Professor Tyndall brought forward opinions of an even bolder character, and avowed his belief that the valleys of