Davis.] 
338 
[Jauuary 18, 
erosion may have done what running water cannot do ; it may 
have excavated rock-basins of no great depth, but sufficient to 
form lakes of moderate size. 
Admitting this final conclusion as a possibility, it remains to 
discover where examples of it actuall/ occur. To demonstrate 
that a lake occupies a rock-basin of glacial erosion is a difficult 
matter ; many rock-basins are produced by subsidence or folding, 
as already noted ; many more that have been called rock-basins 
are really barrier basins as we shall see. The occurrence of solid 
rock in a lake’s outlet is by no means full proof that it forms the 
entire barrier, although often appealed to as such. With the dem- 
onstration that a genuine rock-basin is of glacial erosion, there 
should come also the explanation of the localization of the exces- 
sive erosive force at the point where the lake is now found, and in 
this connection we may look at certain examples brought for-, 
ward by various glacialists. 
The first supposition is that where a glacier is thickest, there it 
will excavate most deeply and form a rock-basin. This seems 
natural enough, but I believe it rests on a misapprehension of the 
character of glacial motion ; and it is seldom possible to adduce 
proof that shall be independent of the fact to be proved : the 
supposition is made to support itself. 
Speaking of Lake Geneva, Ramsay says 1 “ the enormous mass 
of ice scooj>ed out the lake most deeply where the thick- 
ness and weight of ice and consequently its grinding power were 
greatest,” but he gives no proof to show that before the basin was 
scooped out, the ice was thickest where the lake now stands. 
Indeed there is good evidence to show that the highest part of 
the surface of the old glacier of the Rhone was not over Lake 
Geneva, but extended square across the plain in continuation of 
the lower bend of the upper Rhone (from Martigny to Villeneuve) 
and met the Jura at Chasseron, where erratics reach their greatest 
elevation on those mountains ; it is therefore rather on this line 
that the lake should have been cut had its location depended on 
the thickness of the ice . 2 
1 Geol. Soc. Journ., xvm, 1862, 195. 
2 Favre, On the Origin of the Alpine Lakes and Valleys. Phil. Mag., xxix, 1865,208. 
