Sec. 40. The most interesting examples of these cliffs are usually to be seen impendent above strong torrents, which, if forced originally to run in a valley, such as _a_ in Fig. 89, bearing the relation there shown to the inclination of beds on each side, will not, if the cleavage is across the beds, cut their channel straight down, but in an inclined direction, correspondent to the cleavage, as at _b_. If the operation be carried far, so as to undermine one side of the ravine too seriously, the undermined masses fall, partially choke the torrent, and give it a new direction of force, or diminish its sawing power by breaking it among the fallen masses, so that the cliff never becomes very high in such an impendent form; but the trench is hewn downwards in a direction irregularly vertical. Among the limestones on the north side of the Valles, they being just soft enough to yield easily to the water, and yet so hard as to maintain themselves in massy precipices, when once hewn to the shape, there are defiles of whose depth and proportions I am almost afraid to state what I believe to be the measurements, so much do they differ from any which I have seen assigned by scientific men as the limits of precipitous formation. I can only say that my deliberate impression of the great ravine cut by the torrent which descends from the Aletsch glacier, about half way between the glacier and Brieg, was, that its depth is between a _thousand and fifteen hundred_ feet, by a breadth of between _forty and a hundred_. But I could not get to the edge of its cliffs, for the tops rounded away into the chasm, and, of course, all actual measurement was impossible. There are other similar clefts between the Bietschhorn and the Gemmi; and the one before spoken of at Ardon, about five miles below Sion, though quite unimportant in comparison, presents some boldly overhanging precipices easily observed by the passing traveller, as they are close to the road. The glen through which the torrent of the Trient descends into the valley of the Rhone, near Martigny, though not above three or four hundred feet deep, is also notable for its narrowness, and for the magnificent hardness of the rock through which it is cut,--a gneiss twisted with quartz into undulations like those of a Damascus sabre, and as compact as its steel.