The river Rhone rises among the Bernese Alps, and after a course of about 100 miles through the Canton of Valais, it enters the upper end of the Lake of Geneva. Its waters, where they mingle with those of the lake, are muddy and discoloured, but where they pass out at the town of Geneva are limpid and clear. The mud, therefore, which they bring into the lake must be deposited there, and as the stream may have continued to flow for thousands of years, we may reasonably expect to find some trace of the large amount of sediment necessarily deposited during the whole or part of that long period. Accordingly, careful examination of the Lake of Geneva has shown that such accumulations have really been formed, and that their progress and amount during part of the historic period can be approximately calculated. Where the turbid current of the Rhone enters the still water of the lake, the mud slowly sinks to the bottom. In the lapse of centuries layer after layer has been thrown down, rendering the lake at this part sensibly shallower, until a large area or delta has been filled up and converted into a flat alluvial plain. Thus, a town which in the time of the Romans formed a harbour on the water's edge, now stands more than a mile and a half inland. This new-formed land is entirely the work of the stream, and if we could obtain a complete section of it from the surface to the bottom, "we should see a great series of strata, probably from 600 to 900 feet thick (the supposed original depth of the head of the lake), and nearly two miles in length, inclined at a very slight angle." These strata, which are said to have taken about eight centuries to form, "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, according to Saussure, stands six feet lower."[55] If the present conditions continue for a sufficient length of time, the lake may be eventually filled up with mud, sand, and gravel, deposits that would eventually harden by pressure into shale and sandstone. So that the day may yet arrive when the blue waters of the Leman lake shall have passed away, when the Rhone perchance may have ceased to flow or found its way by some other channel, when the peasant may guide the plough where now the boatman plies the oar, and when the geologist shall trace out in quarries and excavations the successive deposits of hardened sediment with their lacustrine shells and drift-wood, and, musing on the changes of which they are the silent yet impressive witnesses, may sit down to pen a record of the gradual extinction of the Leman lake on that classic ground where an immortal historian described the decline and fall of the empire of Rome.