The vast country we have thus described is protected on the north, west, and south by two seas, and by the Pyrenees. On the east, where it is exposed to invasions, Nature, not satisfied with the defences she had given it in the Rhine and the Alps, has further retrenched it behind three groups of interior mountains--first, the Vosges; second, the Jura; third, the mountains of Forez, the mountains of Auvergne, and the Cévennes. The Vosges run parallel to the Rhine, and are like a rampart in the rear of that river. The Jura, separated from the Vosges by _the Gap (trouée) of Belfort_, rises like a barrier in the interval left between the Rhine and the Rhone, preventing, as far as Lyons, the waters of this latter river from uniting with those of the Saône. The Cévennes and the mountains of Auvergne and Forez form, in the southern centre of Gaul, a sort of citadel, of which the Rhone might be considered as the advanced fosse. The ridges of this group of mountains start from a common centre, take opposite directions, and form the valleys whence flow, to the north, the Allier and the Loire; to the west, the Dordogne, the Lot, the Aveyron, and the Tarn; to the south, the Ardèche, the Gard, and the Hérault. The valleys, watered by navigable rivers, presented--thanks to the fruitfulness of their soil and to their easy access--natural ways of communication, favourable both to commerce and to war. To the north, the valley of the Meuse; to the east, the valley of the Rhine, conducting to that of the Saône, and thence to that of the Rhone, were the grand routes which armies followed to invade the south. Strabo, therefore, remarks justly that Sequania (_Franche-Comté_) has always been the road of the Germanic invasions from Gaul into Italy.[42] From east to west the principal chain of the watershed might easily be crossed in its less elevated parts, such as the plateau of Langres and the mountains of Charolais, which have since furnished a passage to the Central Canal. Lastly, to penetrate from Italy into Gaul, the great lines of invasion were the valley of the Rhone and the valley of the Garonne, by which the mountainous mass of the Cévennes, Auvergne, and Forez is turned.