was pushed on one side by the Borgne. The Rhone, however, had no choice, it was obliged to force, and has forced its way over the cone deposited by the Borgne. The Rhine, on the contrary, had the option of running down by Vaduz to Rheinach, and has adopted this course. The watershed between it and the Weisstannen is, however, only about 20 feet in height, and the people of Zurich watch it carefully, lest any slight change should enable the river to return to its old bed. The result of all these changes is that the rivers have altered their courses from those shown in Fig. 43 to their present beds as shown in Fig. 44.

Another interesting case is that of the Upper Engadine (Fig. 45), to which attention. has been called by Bonney and Heim. The fall of the Val Bregaglia is much steeper than that of the Inn, and the Maira has carried off the head-waters of that river away into Italy. The Col was formerly perhaps as far south as Stampa: the Albegna, the Upper Maira, and the stream from the Forgno Glacier, originally belonged to the Inn, but have been captured by the Lower Maira. Their direction still

indicates this; they seem as if they regretted the unwelcome change, and yearned to rejoin their old companions.

Moreover, as rivers are continually cutting back their valleys they must of course sometimes meet. In these cases when the valleys are at different levels the lower rivers have drained the upper ones, and left dry, deserted valleys. In other cases, especially in flatter districts, we have bifurcations, as, for instance, at Sargans, and several of the Italian lakes. Every one must have been struck by the peculiar bifurcation. of the Lakes of Como and Lugano, while a very slight depression would connect the Lake Varese with the

Upper Engadin

Sils

Maloya

Maira

Stampa

V.Bregaglia

Fig. 45.-River system of the
Maloya.

Maggiore, and give it also a double southern end.

Albegna

Silvaplana

ON LAKES

The problem of the origin of Lakes is by means identical with that of Valleys. The latter are due, primarily as a rule to geological causes, but so far as their present condition is concerned, mainly to the action of rain and rivers. Flowing water, however, cannot give rise to lakes.

It is of course possible to have valleys without lakes, and in fact the latter are, now at least, exceptional. There can be no lakes if the slope of the valley is uniform. To what then are lakes due?

Professor Ramsay divides Lakes into three classes:

1. Those due to irregular accumulations of drift, and which are generally quite shallow. 2. Those formed by moraines.

3. Those which occupy true basins scooped by glacier ice out of the solid rock.

To these must, however, I think be added at least one other great class and several minor ones, namely,

4. Those due to inequalities of elevation

or depression.

5. Lakes in craters of extinct volcanoes, for instance, Lake Avernus.

6. Those caused by subsidence due to the removal of underlying soluble rocks, such as some of the Cheshire Meres.

7. Loop lakes in deserted river courses, of which there are many along the course of the Rhine.

8. Those due to rock falls, landslips, or lava currents, damming up the course of a river.

9. Those caused by the advance of a glacier across a lateral valley, such as the Mergelen See, or the ancient lake whose margins form the celebrated "Parallel Roads of Glen Roy."

As regards the first class we find here and there on the earth's surface districts sprinkled with innumerable shallow lakes of all sizes, down to mere pools. Such, for instance, occur in the district of Le Pays de Dombes between the Rhone and the Saône, that of La Sologne near Orleans, in parts of North America, and in Finland. Such lakes are, as a rule, quite

shallow. Some geologists, Geikie, for instance, ascribe them to the fact of these regions having been covered by sheets of ice which strewed the land with irregular masses of clay, gravel, and sand, lying on a stratum impervious to water, either of hard rock such as granite or gneiss, or of clay, so that the rain cannot percolate through it, and without sufficient inclination to throw it off.

2. To Ramsay's second class of Lakes belong those formed by moraines. Such, for instance, are the Lakes of Zürich, Hallwyl, and Sempach. There are in Switzerland many other valleys crossed by old moraines, but they have in most cases generally been worn through by the rivers.

3. Ramsay and Tyndall attribute most of the great Swiss and Italian lakes to the action of glaciers, and regard them as rock basins. The Lake of Geneva, 1230 feet above the sea, is 1000 feet deep; the Lake of Thun is 1856 feet above the sea, and 856 feet deep, that of Constance 770. The Italian Lakes are even more remarkable.