250 
SCENERY OF SWITZERLAND. 
depression leads southwards; it is evident that the 
Jordan once ran into the Gulf of Akaba and so to 
the Red Sea, and that a subsequent change of level 
has created the Dead Sea, which has a depth of 
396 metres below the Ocean level. 
The great American lakes are also probably due 
to differences of elevation. Round Lake Ontario, for 
instance, there is a raised beach which at the western 
end of the lake is no metres above the sea level, 
but rises towards the east and north, until near Fine 
it reaches an elevation of nearly 300 metres. As 
this terrace must have originally been horizontal, we 
have here a lake barrier, due to a difference of ele- 
vation, amounting to over 180 metres. But though 
the lakes may not have been excavated by glaciers 
it is probable that the process of filling up would 
have made much more progress had they not been 
for so long a period occupied by the ice. 
The next question which arises is as to the age 
of the lakes. The valleys are now regarded by most 
Swiss geologists as pre-glacial, but the lakes them- 
selves originated after the retreat of the glaciers.* 
If these views are correct the larger lakes north 
of the Alps may be divided into three classes. 
* Penck, Vergletschernng der Deutsche?i Alpen. 
