LAKES CONNECTED WITH GLACIAL ACTION. 185 
principal cause of lake-basins, is a question still open to dis¬ 
cussion. 
The lakes of Switzerland and the north of Italy are some 
of them twenty and thirty miles in length, and so deep that 
their bottoms are in some cases from 1000 to 2000 feet be¬ 
neath the level of the sea. It is admitted on all hands that 
they were once filled with ice, and as the existing glaciers 
polish and grind down, as before stated, the surface of the 
rocks, we are prepared to find that every lake-basin in coun¬ 
tries once covered by ice should bear the marks of super¬ 
ficial glaciation, and also that the ice during its advance and 
retreat should have left behind it much transported matter 
as well as some evidence of its having enlarged the pre-ex¬ 
isting cavity. But much more than this is demanded by the 
advocates of glacial erosion. They suggest that as the old 
extinct glaciers were several thousand feet thick, they were 
able in some places gradually to scoop out of the solid rock 
cavities twenty or thirty miles in length, and as in the case 
of Lago Maggiore from a thousand to two thousand six hun¬ 
dred feet below the previous level of the river-channel, and 
also that the ice had the power to remove from the cavity 
formed by its grinding action all the materials of the miss¬ 
ing rocks. A constant supply, it is argued, of fine mud is¬ 
sues from the termination of every glacier in the stream 
which is produced by the melting of the ice, and this result 
of friction is exhibited both during winter and summer, af¬ 
fording evidence of the continual deejDening and widening of 
the valleys through which glaciers pass. As the fine mud is 
carried away by a river from the deep pool which is formed 
from the base of every cataract, so it seems to be imagined 
that lake-basins may be gradually emptied of the mud form¬ 
ed by abrasion during the glacial period. 
I am hj no means disposed to object to this theory on the 
ground of the insufficiency of the time during which the ex¬ 
treme cold endured, but we must carefully consider whether 
that same time is not so vast as to make it probable that 
other forces, besides the motion of glaciers, must have co¬ 
operated in converting some parts of the ancient valley 
courses into lake-basins. They who have formed the most 
exalted conceptions of the erosive energy of moving ice do 
not deny that during the period termed “ Glacial ” there have 
been movements of the earth’s crust sufficient to produce os¬ 
cillations of level in Europe amounting to 1000 feet or more 
in both directions. M. Charpentier, indeed, attributed some 
of the principal changes of climate in Switzerland, during the 
glacial period, to a depression of the central Alps to the ex- 
