§ 14. It will be observed also, that the beauty of the crest, in both [Plate 33] and [Fig. 54], depends on the gradually increasing steepness of the lines of slope between a and b. This is in great part deceptive, being obtained by the receding of the crest into a great mountain crater, or basin, as explained in § 11. But this very recession is a matter of interest, for it takes place exactly on the line above spoken of, where the slaty crystallines of the crest join the compact crystallines of the aiguilles; at which junction a correspondent chasm or recession, of some kind or another, takes place along the whole front of Mont Blanc.
§ 15. In the third paragraph of the last chapter we had occasion to refer to the junction of the slaty and compact crystallines at the roots of the aiguilles. It will be seen in the figure there given, that this change is not sudden, but gradated. The rocks to be joined are of the two types represented in [Fig. 3], [p. 106] (for convenience' sake I shall in the rest of this chapter call the slaty rock gneiss, and the compact rock protogine, its usual French name). [Fig. 55] shows the general manner of junction, beds of gneiss occurring in the middle of the protogine, and of protogine in the gneiss; sometimes one touching the other so closely, that a hammer-stroke breaks off a piece of both; sometimes one passing into the other by a gradual change, like the zones of a rainbow; the only general phenomenon being this, that the higher up the hill the gneiss is, the harder it is (so that while it often yields to the pressure of the finger down in the valley, on the Montanvert it is nearly as hard as protogine); and, on the other hand, the lower down the hill, or the nearer the gneiss, the protogine is, the finer it is in grain. But still the actual transition from one to the other is usually within a few fathoms; and it is that transition, and the preparation for it, which causes the great step, or jag, on the flank of the chain, and forms the tops of the Aiguille Bouchard, Charmoz ridges, Tapia, Montagne de la Côte, Montagne de Taconay, and Aiguille du Gouté.
§ 16. But what most puzzled me was the intense straightness of the lines of the gneiss beds, dipping, as it seemed, under the Mont Blanc. For it has been a chief theory with geologists that these central protogine rocks have once been in fusion, and have risen up in molten fury, overturning and altering all the rocks around. But every day, as I looked at the crested flanks of the Mont Blanc, I saw more plainly the exquisite regularity of the slopes of the beds, ruled, it seemed, with an architect's rule, along the edge of their every flake from the summits to the valley. And this surprised me the more because I had always heard it stated that the beds of the lateral crests, a and b, [Fig. 56], varied in slope, getting less and less inclined as they descended, so as to arrange themselves somewhat in the form of a fan. It may be so; but I can only say that all my observations and drawings give an opposite report, and that the beds seemed invariably to present themselves to the eye and the pencil in parallelism, modified only by the phenomena just explained (§§ 9, 10). Thus the entire mass of the Aiguille Bouchard, of which only the top is represented in [Plate 33], appeared to me in profile, as in [Fig. 57], dependent for all its effect and character on the descent of the beds in the directions of the dotted lines, a, b, d. The interrupting space, g g, is the Glacier des Bois; M is the Montanvert; c, c, the rocks under the glacier, much worn by the fall of avalanches, but, for all that, showing the steep lines still with the greatest distinctness. Again, looking down the valley instead of up, so as to put the Mont Blanc on the left hand, the principal crests which support it, Taconay and La Côte, always appeared to me constructed as in [Plate 35] ([p. 212]), they also depending for all their effect on the descent of the beds in diagonal lines towards the left. Nay, half-way up the Breven, whence the structure of the Mont Blanc is commanded, as far as these lower buttresses are concerned, better than from the top of the Breven, I drew carefully the cleavages of the beds, as high as the edge of the Aiguille de Gouté, and found them exquisitely parallel throughout; and again on the Cormayeur side, though less steep, the beds a, b, [Fig. 58], traversing the vertical irregular fissures of the great aiguille of the Allée Blanche, as seen over the Lac de Combal, still appeared to me perfectly regular and parallel.[71] I have not had time to trace them round, through the Aiguille de Bionassay, and above the Col de Bonhomme, though I know the relations of the beds of limestone to the gneiss on the latter col are most notable and interesting. But, as far as was required for any artistical purposes, I perfectly ascertained the fact that, whatever their real structure might be, these beds did appear, through the softer contours of the hill, as straight and parallel; that they continued to appear so until near the tops of the crests; and that those tops seemed, in some mysterious way, dependent on the junction of the gneissitic beds with, or their transition into, the harder protogine of the aiguilles.
Look back to [Plate 33]. The peak of the Bouchard, a, is of gneiss, and its beds run down in lines originally straight, but more or less hollowed by weathering, to the point h, where they plunge under débris. But the point b is, I believe, of protogine; and all the opposed writhing of the waves of rock to the right appears to be in consequence of the junction.
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| 34. Cleavages of Aiguille Bouchard. |