DAVIS: GLACIAL EROSION. 
285 
matured stage of dissection must have been attained in the district 
of the Italian lakes in preglacial time ; for the main valleys are 
widely opened, and even the lateral valleys have flaring slopes. 
In a mature stage of dissection mountains should exhibit a well- 
advanced grading of their slopes ; that is, their sides should be 
worn back to a comparatively even declivity with little regard to 
diversity of structure ; the descending streams of waste being thus 
seen to correspond to the flood plains of graded rivers. The 
agencies of weathering and transportation are delicately balanced 
wherever graded slopes prevail; and a slight tilting of the moun¬ 
tain mass might suffice to disturb the adjustment between the sup¬ 
ply and the removal of waste; then all the steepened slopes would 
soon be more or less completely stripped of their waste cover; 
their rock ledges would be laid bare, although still preserving 
the comparatively even declivity that had been gained under the 
slowly moving waste. 
If the lakes had been formed by warping, it is possible to deduce 
with considerable accuracy the localities where the mountain slopes 
would be steepened and stripped; namely, the northern slopes 
about the southern end of the lakes, and the southern slopes about 
the northern end; but as far as I was able to examine the district 
about Lake Lugano, no effects of such a warping and tilting were 
to be detected. The submergence of lateral valleys about the 
middle of the lakes is also, as has been well pointed out by Wallace, 
a necessary consequence of the theory of warping; but although 
the main valley floor is now deep under water, the side valleys 
are not submerged. Failing to find evidence of warping, and 
being much impressed with the evidence of deep glacial erosion as 
indicated by the hanging lateral valleys of the overdeepened 
Ticino, I examined the irregular troughs of Lake Lugano for similar 
features, and found them in abundance. 
One of the reasons why Lake Lugano had been selected for 
special study was that it did not lie on the line of any master 
valley leading from the central Alps to the piedmont plains ; hence, 
if influenced by ice action at all, its basin must have been less 
eroded than those of Como and Maggiore on the east and west. 
But in spite of this peculiarity of position, Lugano received strong 
ice streams from the great glaciers of the Como and Maggiore 
troughs (see Glacial Distributaries, below), and its enclosing slopes 
possess every sign of having been strongly scoured by ice action. 
The sides of the lake trough are often steep and cliff-like for hun- 
