Note 71 — Chapter XV

Nor did any nearer observations ever induce me to form any contrary opinion. It is not easy to get any consistent series of measurements of the slope of these gneiss beds; for, although parallel on the great scale, they admit many varieties of dip in minor projections. But all my notes unite, whether at the bottom or top of the great slope of the Montanvert and La Côte, in giving an angle of from 60° to 80° with the horizon; the consistent angle being about 75°. I cannot be mistaken in the measurements themselves, however inconclusive observations on minor portions of rock may be; for I never mark an angle unless enough of the upper or lower surface of the beds be smoothly exposed to admit of my pole being adjusted to it by the spirit-level. The pole then indicates the strike of the beds, and a quadrant with a plumb-line their dip; to all intents and purposes accurately. There is a curious distortion of the beds in the ravine between the Glacier des Bois and foot of the Montanvert, near the ice, about a thousand feet above the valley; the beds there seem to bend suddenly back under the glacier, and in some places to be quite vertical. On the opposite side of the glacier, below the Chapeau, the dip of the limestone under the gneiss, with the intermediate bed, seven or eight feet thick, of the grey porous rock which the French call cargneule, is highly interesting; but it is so concealed by débris and the soil of the pine forests, as to be difficult to examine to any extent. On the whole, the best position for getting the angle of the beds accurately, is the top of the Tapia, a little below the junction there of the granite and gneiss (see notice of this junction in Appendix 2); a point from which the summit of the Aiguille du Gouté bears 11° south of west, and that of the Aiguille Bouchard 17° north of east, the Aiguille Dru 5-½° or 6° north of east, the peak of it appearing behind the Petit Charmoz. The beds of gneiss emerging from the turf under the spectator's feet may be brought parallel by the eye with the slopes of the Aiguille du Gouté on one side, and the Bouchard (and base of Aiguille d'Argentière) on the other; striking as nearly as possible from summit to summit through that on which the spectator stands, or from about 10° north of east to 10° south of west, and dipping with exquisite uniformity at an angle of 74 degrees with the horizon. But what struck me as still more strange was, that from this point I could distinctly see traces of the same straight structure running through the Petit Charmoz, and the roots of the aiguilles themselves, as in Fig. 59; nor could I ever, in the course of countless observations, fairly determine any point where this slaty structure altogether had ceased. It seemed only to get less and less traceable towards the centre of the mass of Mont Blanc; and, from the ridge of the Aiguille Bouchard itself, at the point a in Plate 33, whence, looking south-west, the aiguilles can be seen in the most accurate profile obtainable throughout the valley of Chamouni, I noticed a very singular parallelism even on the south-east side of the Charmoz, x y (Fig. 60), as if the continued influence of this cleavage were carried on from the Little Charmoz, c, d (in which, seen on the opposite side, I had traced it as in Fig. 59), through the central mass of rock r. In this profile, M is the Mont Blanc itself; m, the Aiguille du Midi; P, Aiguille du Plan; b, Aiguille Blaitière; C, Great Charmoz; c, Petit Charmoz; E, passage called de l'Etala.