its present form, because the river may have flowed into it for
thousands of years, without importing any sediment whatever. Such would
have been the case, if the waters had first passed through a chain of
upper lakes; and that this was actually the fact, seems indicated by the
course of the Rhone between Martigny and the Lake of Geneva, and, still
more decidedly, by the channels of many of its principal feeders.

If we ascend, for example, the valley through which the Dranse flows, we
find that it consists of a succession of basins, one above the other, in
each of which there is a wide expanse of flat alluvial lands, separated
from the next basin by a rocky gorge, once perhaps the barrier of a
lake. The river seems to have filled these lakes, one after the other,
and to have partially cut through the barriers, some of which it is
still gradually eroding to a greater depth. Before, therefore, we can
pretend even to hazard a conjecture as to the era at which the principal
delta of Lake Leman or any other delta commenced, we must be thoroughly
acquainted with the geographical features and geological history of the
whole system of higher valleys which communicate with the main stream,
and all the changes which they have undergone since the last series of
convulsions which agitated and altered the face of the country.

_Lake Superior._--Lake Superior is the largest body of freshwater in the
world, being above 1700 geographical miles in circumference when we
follow the sinuosities of its coasts, and its length, on a curved line
drawn through its centre, being more than 400, and its extreme breadth
above 150 geographical miles. Its surface is nearly as large as the
whole of England. Its average depth varies from 80 to 150 fathoms; but,
according to Captain Bayfield, there is reason to think that its
greatest depth would not be overrated at 200 fathoms, so that its bottom
is, in some parts, nearly 600 feet below the level of the Atlantic, its
surface being about as much above it. There are appearances in different
parts of this, as of the other Canadian lakes, leading us to infer that
its waters formerly occupied a higher level than they reach at present;
for at a considerable distance from the present shores, parallel lines
of rolled stones and shells are seen rising one above the other, like
the seats of an amphitheatre. These ancient lines of shingle are exactly
similar to the present beaches in most bays, and they often at