28(1 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
I 
dreds of feet above present water level, thus simulating the basal 
cliffs of the Ticino valley; while at greater heights the valley sides 
lean back in relatively well-graded slopes, as in Plate 1, Figure B, 
where the southern side of the northeastern arm of the lake, near 
Porlezza, is shown. The angle at the change of slope is often well 
defined, but it is independent of rock structure. Narrow ravines are 
frequently incised in the basal cliffs, and alluvial fans of greater or 
less size are built into the lake waters from the base of the ravines. 
The northeastern arm of the lake, extending from the town 
of Lugano to Porlezza, receives several cascading streams from 
hanging valleys on its southern side, one of which is here shown in 
Plate 2, Figure A. The side slopes of the hanging valleys are for the 
most part flaring open and well graded, from which it must be con¬ 
cluded that their streams had, under some condition no longer exist¬ 
ing, ceased to deepen their valleys for a time long enough to allow the 
valley sides to assume a mature expression; and that since then the 
bottom trough of the main arm of the lake has been eroded deep and 
wide, with a very small accompanying change in the lateral valleys. 
In other words, the side valleys were, in preglacial time, eroded to 
a depth accordant with the floor of the master valley that they 
joined, and since then the bottom trough has been eroded in the 
floor of the master valley by a branch of the Como glacier. In 
postglacial time the side streams have begun to trench their valley 
floors, eroding little canyons; but much of this sort of work must 
be done before the side valleys are graded down even to the level 
of the lake waters, much less to the level of the bottom of the lake. 
The two southern arms of the lake lead to troughs whose floors 
ascend southward to the moraines of the foot-hills, beyond which 
stretch forward the abundant overwashed gravels of the great plain 
of the Po. 
I do not mean to imply that every detail of form about Lake 
Lugano can find ready explanation by the mature glacial modifi¬ 
cation of a mature preglacial valley system ; but a great number of 
forms may be thus explained, and a belief in strong glacial erosion 
was forced upon me here as well as in the valley of the Ticino. 
A detailed study of the Italian lakes with the intention of care¬ 
fully sorting out all the glacial modifications of preglacial forms 
would be most profitable. 
Various Examples of Glaciated Valleys .—My excursions of 
last summer showed me a number of over-deepened main valleys 
and hanging lateral valleys in the Alps; for example, those of the 
