The basin of the Rhine is separated from that of the Rhone by the Monts Faucilles, the southern extremity of the Vosges, called _Le trouée de Belfort_, the Jura, the Jorat (the heights which surround the Lake of Geneva on the north), and the lofty chain of the Helvetic Alps. In its upper part, it embraces nearly all Switzerland, of which the Rhine forms the northern boundary, in its course, from east to west, from the Lake of Constance to Bâle. Near this town the river turns abruptly towards the north. The basin widens, limited to the east by the mountains which separate it from the Danube and the Weser; to the west, by the northern part of the great line of watershed (the mountains of the Meuse, the Argonne, and the western Ardennes). It is intersected, from Mayence to Bonn, by chains nearly parallel to the course of the river, which separate its tributaries. From Bonn to the point where the Rhine divides into two arms, the basin opens still more; it is flat, and has no longer a definite boundary. The southern arm bore already, in the time of Cæsar, the name of _Waal_ (Vahalis), and united with the Meuse[41] below Nimeguen. To the west of the basin of the Rhine, the Scheldt forms a secondary basin. The basin of the Rhone, in which is comprised that of the Saône, is sharply bounded on the north by the southern extremity of the Vosges and the Monts Faucilles; on the west, by the plateau of Langres, the Côte-d’Or, and the Cévennes; on the east, by the Jura, the Jorat, and the Alps. The Rhone crosses the Valais and the Lake of Geneva, follows an irregular course as far as Lyons, and runs thence from north to south to the Mediterranean. Among the most important of its secondary basins, we may reckon those of the Aude, the Hérault, and the Var. The three great basins of the western slope are comprised between the line of watershed of Gaul and the ocean. They are separated from each other by two chains branching from this line, and running from the south-east to the north-west. The basin of the Seine, which includes that of the Somme, is separated from the basin of the Loire by a line of heights which branches from the Côte-d’Or under the name of the mountains of the Morvan, and is continued by the very low hills of Le Perche to the extremity of Normandy. A series of heights, extending from north to south, from the hills of Le Perche to Nantes, enclose the basin of the Loire to the west, and leave outside the secondary basins of Brittany. The basin of the Loire is separated from that of the Garonne by a long chain starting from Mont Lozère, comprising the mountains of Auvergne, those of the Limousin, the hills of Poitou, and the plateau of Gatine, and ending in flat country towards the coasts of La Vendée. The basin of the Garonne, situated to the south of that of the Loire, extends to the Pyrenees. It comprises the secondary basins of the Adour and the Charente.