THE LAKE OF GENEVA. 
9> 
at a depth of 309 metres, indicating that the alluvium 
must be of great depth. 
The western half of the lake is in almost horizontal 
strata of middle Miocene Mollasse. It was therefore 
excavated after the middle Miocene, and before the 
close of the Glacial epoch. 
As already mentioned, there is some reason for 
supposing that the Petit Lac was originally the valley 
of the Arve. It presents a general inclination from 
Geneva to Merges, but with some slightly marked 
basins, owing to transverse banks, which Forel con- 
siders to be ancient moraines. The sides, moreover, 
like those of an ordinary river valley, slope more or 
less towards the centre. I have already {ante, vol. i. 
p. 227) given reasons for thinking that the outflow ot 
the waters was formerly, not at Geneva, but between 
Merges and Lausanne, to the Lake of Neuchatel. 
The following figure gives the profile from St. Prex 
to Amphion. 
It must always be remembered that the vertical 
and horizontal scales in this and other similar figures 
are quite different. They bring out clearly the special 
point which they are intended to illustrate, but in 
other respects might give an erroneous impression. 
We generally think of the Lake of Geneva as deep. 
