1882 .] 
341 
[Davis. 
Scotland known as the Great Glen. The North Channel may 
surely be partly at least the result of stratigraphic displacement, 
and of subsidence in a region so very volcanic. Not proven must 
be the verdict here. 
It has been suggested 1 that lakes are formed where the valley 
down which the glacier advanced decreased its slope on reaching 
a more level country, because there would be an increase of thick- 
ness and of pressure at the point of change. Certainly many 
lakes are thus placed, but exceptions are equally numerous, and 
in many of the examples claimed as rock-basins, this important 
point of structure is' insufficiently proved. 
Local weakness of the rocks over which the ice moved may 
have determined many of the smaller basins, but it is as insuffi- 
cient as the other suggestions in explaining many of the larger 
lakes. Lake Geneva is not deepest at its western end where it is 
cut in soft rocks, but near its eastern where it lies in hard lime- 
stones. So the marginal lakes of the Alps from Thun to Wal- 
lenstadt, are, as above noted, in hard rocks that have been recently 
flooded by back-water and not in the softer rocks of the plain 
on which their glaciers extended. Our Great Lakes generally 
follow limestones and softer rocks very closely, but the evidence 
ordinarily given to show that the basins were excavated by gla- 
ciers really proves nothing more than that they were for a time 
occupied by glaciers ; there are many facts that lead us to suppose 
the greater part of these lakes lie in old valleys, clogged by drift. 
As suggested by Kinahan , 2 the local weakness which determined 
the site of a lake is the result of faults and fissures intersect- 
ing, and breaking the rock into fragments so it can easily be 
scraped out ; this may be sometimes the case, but as a rule I 
believe the effect of faults to be less marked. Many of the lakes 
mentioned by this author may be found to be in barrier basins. 
The- difficulties that lie in the way of accepting a glacial origin 
for the larger lakes are, in addition to the insufficiency of gla- 
cial erosion, first, that the necessity for this acceptance is not 
1 Geikie, op. cit. 278, quoting Seue, Le Ndvd de Justedal et ses Glaciers. An 
enormously exaggerated diagram showing this supposition is given by A. Tylor, 
Geol. Mag., ii, 1875, 438. 
2 Valleys, etc. (See our p. 329.) 
