284 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
of the Linth and the Wallen See have been cut down and broadly 
opened in the same limestones. If the troughs were of normal 
river origin, the side streams also should -have by this time trenched 
the limestones deeply, instead of falling over the limestone cliffs at 
the very side of the larger troughs. In the Ticino valley where 
the side streams are most discordant, massive gneisses prevail; the 
structure is so nearly uniform over large areas that it affords no 
explanation of the strong discordance between side and main 
valleys. 
It thus seems obligatory to conclude that the bottom troughs of 
the larger Alpine valleys were deepened and widened by ice action. 
This belief is permitted by the abundant signs of glacial erosion on 
the spurless basal cliffs, and required by the persistent association 
of over-deepened bottom troughs and discordant hanging lateral 
valleys with regions of strong glaciation. The valley of the Ticino 
manifests these peculiarities very distinctly, and I have recently 
described them in some detail in a paper in Appalachia ( 1900 ). 
Subaerial Erosion during the Glacial Period. — It should not 
be imagined that the glacial erosion of troughs in valley floors was 
necessarily so rapid that no significant subaerial erosion was accom¬ 
plished during its progress. Ordinary weathering and down-hill 
transportation of rock waste must have been in active operation on 
the valley sides above the border of the ice-filled channels; and 
the very fact that on the upper slopes of the mountains, pre- 
glacial, glacial and postglacial erosion was similarly conditioned, 
makes it difficult to distinguish the work done there in each of these 
three chapters of time. In the diagrams accompanying this article 
no indication of change from preglacial to postglacial outline on 
the upper mountain slopes is indicated, because no satisfactory meas¬ 
ure can be given to it. 
Lake Ijugano. — In the presence of a variety of evidence col¬ 
lected for some years previous to my recent European trip, it had 
been my feeling that the best explanation offered for the large lakes 
that occupy certain valleys on the Italian slope of the Alps was 
that they had resulted from what has been called valley-warping, as 
set forth by Lyell, Heim and others. It was my desire to look es¬ 
pecially at Lakes Maggiore, Lugano and Como with this hypothesis 
in mind, and to subject it to a careful test by means of certain asso¬ 
ciated changes that should expectedly occur on the slopes of the 
neighboring mountains, as may be explained as follows. 
On the supposition of moderate or small glacial erosion, a well- 
