was going on during a large part of the Glacial period, under a weight of ice varying from one to three or four thousand feet in thickness; that the huge grinding tool was at work day and night, winter and summer, century after century, for whatever number of thousands of years we give to the Glacial period; that, as innumerable other facts prove, the ice moved irresistibly over hill and dale, and up slopes far steeper than any formed by the upward slopes of the bottom of our deepest lakes, what is there of impossible, or even of improbable, in the belief that lake basins were produced by such differential erosion? To the ordinary observer it seems impossible that a mountain valley, half a mile wide and bounded by rocky slopes and precipices two or three thousand feet high, can have been formed without any "convulsion of Nature," but merely by the natural agencies he sees still in action—rain and frost, sun and wind—and that the small, rock-encumbered stream now flowing along its bottom can have carried away the whole of the cubic miles of solid rock that once filled up the valley. But the geologist knows that these apparently insignificant forces have done the work, through their continuous action always in one direction for thousands or even for millions of years; and, therefore, having before him so many proofs of the eroding power of ice, in planed and rounded rocks, and in the grooves and furrows which are the latest marks left by the ice tool, and bearing in mind the long duration and possibly recurrent phases of the Ice age—to be measured certainly by tens, perhaps by hundreds of thousands of years—he can have little difficulty in accepting the erosion of lake basins as the most satisfactory explanation of their origin.
Objections of Modern Writers considered.—Prof. Bonney and many other writers ask, why lakes are so few though all the chief valleys of the Alps were filled with ice; and why, for instance, there is no great lake in the Dora Baltea Valley, whose glacier produced the great moraines of Ivrea opposite its outlet into the plains of Italy, and which form a chain of hills fifteen miles long and fifteen hundred feet high. The answer, in the case of the Dora Baltea, is not difficult, since it almost certainly has had a series of lake basins at Aosta, Verrex, and other places where the broad, level valley is now filled with alluvial gravel. But the more important point is the extreme narrowness of the lower part of the valley above Donnas and again near its entrance into the valley of the Po. The effect of this would be that the great glacier, probably two thousand feet thick or more, would move rapidly in its upper layers, carrying out its load of stones and debris to form the terminal moraine, while the lower strata, choked in the defiles, would move very slowly. And once out in the open valley of the Po, then a great inlet of the warm Medi-
