466 Professor T. G. Bonney—Presidential Address— 
of the larger valleys ; but their lips generally are less deeply notched. 
Whatever may have been the cause, this rapid change in slope must 
indicate a corresponding change of action in the erosive agent. Here 
and there the apex of the V may be slightly flattened, but any 
approach to a real U is extremely rare. The retention of the more 
open form in many small, elevated recesses, from which at the present 
day but little water descends, suggests that where one of them soon 
became buried under snow, but was insignificant as a feeder of a glacier, 
erosion has been for ages almost at a standstill. 
The V-like lower portion in the section of one of the principal 
valleys, which is all that some other observers have claimed for the 
work of a glacier, cannot be ascribed to subsequent modification by 
water, because ice-worn rock can be seen in many places, not only 
high up its sides, but also down to within a yard or two of the present 
torrent. 
Thus valley after valley in the Alps seems to leave no escape from 
the following dilemma: Either a valley cut by a glacier does not 
differ in form from one made by running water, or one which has been 
excavated by the latter, if subsequently occupied, is but superficially 
modified by ice. 
Many lake-basins have been ascribed to the erosive action of glaciers. 
Since the late Sir A. Ramsay advanced this hypothesis numbers of 
lakes in various countries have been carefully investigated and the 
results published, the most recent of which is the splendid work on 
the Scottish lochs by Sir J. Murray and Mr. L. Pullar.* 
Even these latest researches have not driven me from the position 
which J have maintained from the first—namely, that while many 
tarns in corries and lakelets in other favourable situations are 
probably due to excavation by ice, as in the mountainous districts of 
Britain, in Scandinavia, or in the higher parts of the Alps, the 
difficulty of invoking this agency increases with the size of the basin— 
as, for example, in the case of Loch Maree or the Lake of Annecy—till 
it becomes insuperable. Even if Glas Llyn and Llyn Llydaw were 
the work of a glacier, the rock-basins of Gennesaret and the Dead Sea, 
still more those of the great lakes in North America and in Central 
Africa, must be assigned to other causes. 
I pass on, therefore, to mention another difficulty in this hypothesis 
—that the Alpine valleys were greatly deepened during the Glacial 
Epoch—which has not yet, I think, received sufficient attention. 
From three to four hundred thousand years have elapsed, according to 
Penck and Briickner, since the first great advance of the Alpine ice. 
One of the latest estimates of the thickness of the several geological 
formations assigns 4000 feet? to the Pleistocene and Recent, 135,000 
to the Pliocene, and 14,000 to the Miocene. If we assume the times 
of deposit to be proportional to the thicknesses, and adopt the larger 
figure for the first-named period, the duration of the Pliocene would 
1 Bathymetrical Survey of the Scottish Freshwater Lochs, by Sir J. Murray and 
Mr. L. Pullar, 1910. 
2 T have doubts whether this is not too great. 
