III. The distance from Lake Léman to the Jura, following the sinuosities of the river, is 29½ kilomètres, or 19,000 Roman paces (_millia passuum decem novem_).[163] It is on the space comprised between these two points that a retrenchment was raised which is called in the “Commentaries” _murus fossaque_. This could not be a continuous work, as the ground to be defended is intersected by rivers and ravines, and the banks of the Rhone are almost everywhere so precipitous that it would have been useless to fortify them. Cæsar, pressed for time, can only have made retrenchments on the weakest points of the line where the passage of the river was easy; indeed, this is what Dio Cassius tells us.[164] The labours of the Romans were only supplementary, on certain points, to the formidable natural obstacles which the Rhone presents in the greater part of its course. The only places where an attempt could be made to pass it, because the heights there sink towards the banks of the river into practicable declivities, are situated opposite the modern villages of Russin, Cartigny, Avully, Chancy, and Cologny. In these places they cut the upper part of the slope into a perpendicular, and afterwards hollowed a trench, the scarp of which thus gained an elevation of sixteen feet. These works, by uniting the escarpments of the Rhone, formed, from Geneva to the Jura, a continuous line, which presented an impassable barrier. Behind and along this line, at certain distances, posts and closed redoubts rendered it impregnable. (_See Plate 3._)[165] This retrenchment, which required only from two to three days’ labour, was completed when the deputies returned, at the time appointed, to hear Cæsar’s reply. He flatly refused the passage, declaring that he would oppose it with all his means. Meanwhile the Helvetii, and the people who took part in their enterprise, had assembled on the right bank of the Rhone. When they learnt that they must renounce the hope of quitting their country without opposition, they resolved to open themselves a passage by force. Several times--sometimes by day, and sometimes by night--they crossed the Rhone, some by fording, others with the aid of boats joined together, or of a great number of rafts of timber, and attempted to carry the heights, but, arrested by the strength of the retrenchment (_operis munitione_), and by the efforts and missiles of the soldiers who hastened to the threatened points (_concursu et telis_), they abandoned the attack.[166] [Sidenote: The Helvetii begin their March towards the Saône. Cæsar unites his Troops.]