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 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 Silvaplana Upper Engadin Sils Maloy a Maira Stampa V.Bregaglia Albegna depression would connect Fig. 45.- River system of the the Lake Varese with the Maloya. Maggiore, and give it also a double southern end. ON LAKES The problem of the origin of Lakes is by no 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. then are lakes due? To what 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 rockfalls, 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 Doubs 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. The materials forming moraines being, however, comparatively loose, are easily cut through by streams. There are in Switzerland many cases of valleys crossed by old moraines, but they have generally been long ago 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. It is of course obvious that rivers cannot make basin-shaped hollows surrounded by rock on all sides. The Lake of Geneva, 1230 feet above the sea, is over 1000 feet deep; the Lake of Brienz is 1850 feet above the sea, and 2000 feet deep, so that its bottom is really below the sea level. The Italian Lakes are even more remarkable. The Lake of Como, 700 feet above the sea, is 1929 feet deep. Lago Maggiore, 685 feet above the sea, is no less than 2625 feet deep. If the mind is at first staggered at the magnitude of the scale, we must remember that the ice which is supposed to have scooped out the valley in which the Lake of Geneva now reposes, was once at least 4000 feet thick; while the moraines were also of gigantic magnitude, that of Ivrea, for instance, being no less than 1500 feet above the river, and several miles long. Indeed it is obvious that a glacier many hundred, or in some cases several thousand, feet in thickness, must exercise great pressure on the bed over which it travels. We see this from the striæ and grooves on the solid rocks, and the fine mud which is carried down by glacial streams. The deposit The deposit of glacial rivers, the "loess" of the Rhine itself, is mainly the result of this ice-waste, and that is |