JURA, a system of mountain ranges between the Rhine and the Rhone, and forming the frontier between France and Switzer land. The Rhine gorge cuts off the Jura from the Swabian and Franconian ranges to the north-east, and the Rhone gorge, the Jura from the Dauphine to the south. Its three chief rivers—the Doubs, the Loue and the Ain—flow down the western slope, which is much longer and half as steep as the eastern. Some geographers extend the name Jura to the Swabian and Franconian ranges between the Danube, the Neckar and the Main; but, though these are similar to the range to the south, it is most convenient to limit the name of the mountain ridges lying between France and Switzerland, and this narrower sense will be adopted here.
The Jura system is 156 m. long and 38 m. broad, oblong in shape, and raised to an average height of from 1,95o to 2,600 ft. above the surrounding plains. The Juras form essentially a series of ridges parallel to the direction of greatest length of the system. Many transverse gorges or "cluses" cut across these ridges.
Geologically the Jura Mountains belong to the Alpine system; and the same forces which built up the latter produced the folds and faults in the former (see ALPS). The rocks of the Juras range from the Permian to the Pliocene, but the Jurassic forma tion is the most important. The rocks have been folded into a series of parallel anticlines and synclines with a direction from south-west to north-east, but owing to the resistance of the ancient massifs of the central plateau of France on the west and the Vosges-Black Forest on the east the f olds are bunched together at the extremities of the range. The intensity of folding decreases northward, away from the Alps. In the east the structure is complicated by block faulting and the beds here are not folded as a general rule. This latter region is known as the Jura table land (Jura tabulaire) while the greater part of the range is known as the folded Jura (Jura plissé). In the extreme west the moun tains descend steeply to the plain of the SaOne, the district being known as the Region du vignoble.
Countless blocks of gneiss, granite and other crystalline rocks are found on the slopes of the Jura Mountains. These erratic blocks are of glacial origin having been transported to their present position by glaciers from the Alps which have also left their mark on the Jura range itself in the shape of striations and moraines.
In considering the topography of the Jura, it may be well to take a brief survey of its outer slopes.
I. The northern face dominates on one side the famous "Trouee" (or Trench) of Belfort, one of the great geographical centres of Europe, whence routes run north down the Rhine to the North Sea, south-east to the Danube basin and Black Sea, and south-west into France, and so to the Mediterranean basin. It is
of great strategical importance. On the other side it overhangs the "Trouee" of the Black Forest towns on the Rhine (Rhein felden, Sackingen, Laufenburg and Waldshut), through which the central plain of Switzerland is easily gained. On this north slope two openings offer routes into the interior of the chain—the valley of the Doubs (France), and the valley of the Birse (Switzerland). Belfort is the military, Millhausen the industrial, and Basle the commercial centre of this slope.
2. The eastern and western faces offer many striking parallels. The plains through which flow the Aar and the Saone have each been the bed of an ancient lake, traces of which remain in the lakes of Neuchatel, Bienne and Morat. Both east and west slopes are pierced by many transverse gorges, "cluses" by which access is gained to the great central plateau of Pontarlier, though these are seen more plainly on the east face than on the west ; thus the gorges at the exit from which Lons-le-Saunier, Poligny, Arbois and Salins are built balance those of the Suze, of the Val de Ruz, of the Val de Travers, and of the Val d'Orbe. Neuchatel which commands these eastern gorges is a great military and industrial centre, just as is Besancon on the west. These easy means of communication account for the dialect of Neuchatel being Bur gundian. Mt. Chasseron (5,286 ft.), the central point of the eastern face, commands the two great railways which join Neu chatel and Pontarlier. The R. Loue which drains the west face of this ridge flows into the Doubs south of Dole, the only important town of the central portion of the SaOne basin. The Chasseron is wholly Swiss, as are the lower summits of Chasseral (5,279 f t.), Mont Suchet (5,220 ft.), Aiguille de Baulmes (5,128 ft.), Dent de Vaulion (4,879 ft.), Weissenstein (4,245 ft.), and Chau mont (3,862 ft.), the two last-named points being probably the best-known points in the Jura, as they are accessible by carriage road from Soleure (Solothurn) and Neuchatel respectively. South of the Orbe valley the east face becomes a rocky wall which is crowned by all the highest summits (the first and second Swiss, the rest French) of the chain—the Mont Tendre (5,521 f t.), the Dole ( 5,511 ft.), the Reculet (5,643 f t.), the Cret de la Neige (5,653 ft.) and the Grand Credo (5,328 ft.), the uniformity of level being as striking as on the west edge of the Jura, though there the absolute height is far less. The position of the Dole is similar to that of the Chasseron, as along the sides of it run the great roads of the Col de St. Cergues (4,o51 ft.) and the Col de la Faucille ft.), the latter leading through the Vallee des Dappes. The height of these roads shows that they are passages across the chain, rather than through natural depressions.