--Lakes exemplify the first reproductive operations in
which rivers are engaged when they convey the detritus of rocks and the
ingredients of mineral springs from mountainous regions. The accession
of new land at the mouth of the Rhone, at the upper end of the Lake of
Geneva, or the Leman Lake, presents us with an example of a considerable
thickness of strata which have accumulated since the historical era.
This sheet of water is about thirty-seven miles long, and its breadth is
from two to eight miles. The shape of the bottom is very irregular, the
depth having been found by late measurements to vary from 20 to 160
fathoms.[335] The Rhone, where it enters at the upper end, is turbid and
discolored; but its waters, where it issues at the town of Geneva, are
beautifully clear and transparent. An ancient town, called Port Vallais
(Portus Valesiae of the Romans), once situated at the water's edge, at
the upper end, is now more than a mile and a half inland--this
intervening alluvial tract having been acquired in about eight
centuries. The remainder of the delta consists of a flat alluvial plain,
about five or six miles in length, composed of sand and mud, a little
raised above the level of the river, and full of marshes.

Sir Henry De la Beche found, after numerous soundings in all parts of
the lake, that there was a pretty uniform depth of from 120 to 160
fathoms throughout the central region, and on approaching the delta, the
shallowing of the bottom began to be very sensible at a distance of
about a mile and three quarters from the mouth of the Rhone; for a line
drawn from St. Gingoulph to Vevey gives a mean depth of somewhat less
than 600 feet, and from that part of the Rhone, the fluviatile mud is
always found along the bottom.[336] We may state, therefore, that the
new strata annually produced are thrown down upon a slope about two
miles in length; so that, notwithstanding the great depth of the lake,
the new deposits are inclined at so slight an angle, that the dip of the
beds would be termed, in ordinary geological language, horizontal.

The strata probably consist of alternations of finer and coarser
particles; for, during the hotter months from April to August, when the
snows melt, the volume and velocity of the river are greatest, and large
quantities of sand, mud, vegetable matter, and drift-wood are
introduced; but during the rest of the year, the influx is comparatively
feeble, so much so, that the whole lake