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.
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.
Gaul presented the same contrast of climates which we observe between the north and south of France. While the Roman province enjoyed a mild temperature and an extreme fertility,[43] the central and northern part was covered with vast forests, which rendered the climate colder than it is at present;[44] yet the centre produced in abundance wheat, rye, millet, and barley.[45] The greatest of all these forests was that of the Ardennes. It extended, beginning from the Rhine, over a space of two hundred miles, on one side to the frontier of the Remi, crossing the country of the Treviri; and, on another side, to the Scheldt, across the country of the Nervii.[46] The “Commentaries” speak also of forests existing among the Carnutes,[47] in the neighbourhood of the Saône,[48] among the Menapii[49] and the Morini,[50] and among the Eburones.[51] In the north the breeding of cattle was the principal occupation,[52] and the pastures of Belgic Gaul produced a race of excellent horses.[53] In the centre and in the south the richness of the soil was augmented by productive mines of gold, silver, copper, iron, and lead.[54]
The country was, without any doubt, intersected by carriage roads, since the Gauls possessed a great number of all sorts of wagons,[55] since there still remain traces of Celtic roads, and since Cæsar makes known the existence of bridges on the Aisne,[56] the Rhone,[57] the Loire,[58] the Allier,[59] and the Seine.[60]