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On the Old Road Volume 2 (of 2), essay(s) by John Ruskin |
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Notes On Natural Science - The Strata Of Mont Blanc. 1834 |
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_ FACTS AND CONSIDERATIONS ON THE STRATA OF MONT BLANC, AND ON SOME INSTANCES OF TWISTED STRATA OBSERVABLE IN SWITZERLAND.[27] THE INDURATION OF SANDSTONE. 1836 270. The granite ranges of Mont Blanc are as interesting to the geologist as they are to the painter. The granite is dark red, often inclosing veins of quartz, crystallized and compact, and likewise well-formed crystals of schorl. The average elevation of its range of peaks, which extends from Mont Blanc to the Tete Noire, is about 12,000 English feet above the level of the sea. [The highest culminating point is 15,744 feet.] The Aiguille de Servoz, and that of Dru, are excellent examples of the pyramidal and spiratory formation which these granite ranges in general assume. They rise out of immense fields of snow, but, being themselves too steep for snow to rest upon, form red, bare, and inaccessible peaks, which even the chamois scarcely dares to climb. Their bases appear sometimes abutted (if I may so speak) by mica slate, which forms the southeast side of the Valley of Chamonix, whose flanks, if intersected, might appear as (in _fig._ 72), _a_, granite, forming on the one side (B) the Mont Blanc, on the other (C) the Mont Breven; _b_, mica slate resting on the base of Mont Blanc, and which contains amianthus and quartz, in which capillary crystals of titanium occur; _c_, calcareous rock; _d_, alluvium, forming the Valley of Chamonix. I should have mentioned that the granite appears to contain a small quantity of gold, as that metal is found among the granite debris and siliceous sand of the river Arve [_Bakewell_, i. 375]; and I have two or three specimens in which chlorite (both compact and in minute crystals) occupies the place of mica. J. R. _March_, 1834. * * * * * With this paper were printed some observations on it by the Rev. W. B. Clarke, after which (p. 648) appears the following note by J. R. * * * * * 271. "TWISTED STRATA.--The contortions of the limestone at the fall of the Nant d'Arpenaz, on the road from Geneva to Chamonix, are somewhat remarkable. The rock is a hard dark brown limestone, forming part of a range of secondary cliffs, which rise from 500 feet to 1000 feet above the defile which they border. The base itself is about 800 feet high. The strata bend very regularly except at _e_ and _f_,[28] where they appear to have been fractured. * * * * * _To what Properties in Nature is it owing that the Stones in Buildings, formed originally of the frailest Materials, gradually become indurated by Exposure to the Atmosphere and by Age, and stand the Wear and Tear of Time and Weather every bit as well, in some instances much better, than the hardest and most compact Limestones and Granite?_[29] 272. In addition to the fact mentioned by Mr. Hunter[30] relative to the induration of soft sandstone, I would adduce an excellent example of the same effect in the cathedral of Basle, in Switzerland. The cathedral is wholly built of a soft coarse-grained sandstone, of so deep a red as to resemble long-burned brick. The numerous and delicate ornaments and fine tracery on the exterior are in a state of excellent preservation, and present none of the moldering appearance so common in old cathedrals that are built of stone which, when quarried, was much harder than this sandstone. The pavement in the interior is composed of the same material; and, as almost every slab is a tomb, it is charged with the arms, names, and often statues in low relief, of those who lie below, delicately sculptured in the soft material. Yet, though these sculptures have been worn for ages by the feet of multitudes, they are very little injured; they still stand out in bold and distinct relief: not an illegible letter, not an untraceable ornament is to be found; and it is said, and I believe with truth, that they have now grown so hard as not to be in the least degree farther worn by the continual tread of thousands; and that the longer the stone is exposed to the air, the harder it becomes. The cathedral was built in 1019. 273. The causes of the different effects of air on stone must be numerous, and the investigation of them excessively difficult. With regard, first, to rocks _en masse_, if their structure be crystalline, or their composition argillaceous, the effect of the air will, I think, ordinarily, be found injurious. Thus, in granite, which has a kind of parallelogrammatic cleavage, water introduces itself into the fissures, and the result, in a sharp frost, will be a disintegration of the rocks _en masse_; and, if the felspar be predominant in the composition of the granite, it will be subject to a rapid decomposition. The morvine of some of the Chamouni and Allee Blanche glaciers is composed of a white granite, being chiefly composed of quartz and felspar, with a little chlorite. The sand and gravel at the edge of these glaciers appears far more the result of decomposition than attrition. All finely foliated rocks, slates, etc., are liable to injury from frost or wet weather. The road of the Simplon, on the Italian side, is in some parts dangerous in, or after, wet weather, on account of the rocks of slate continually falling from the overhanging mountains above; this, however, is mere disintegration, not decomposition. Not so with the breccias of Central Switzerland. The rock of Righi is composed of pebbles of different kinds, joined by a red argillaceous gluten. When this rock has not been exposed to the air, it is very hard: you may almost as easily break the pebbles as detach them from their matrix; but, when exposed for a few years to wind and weather, the matrix becomes soft, and the pebbles may be easily detached. I was struck with the difference between this rock and a breccia at Epinal, in France, where the matrix was a red sandstone, like that of the cathedral at Basle. Here, though the rock had every appearance of having been long exposed to the air, it was as hard as iron; and it was utterly impossible to detach any of the pebbles from the bed: it was difficult even to break the rock at all. I cannot positively state that the gluten in these sandstones is calcareous, but I suppose it to have been so. Compact calcareous rock, as far as I remember, appears to be subject to no injury from the weather. Many churches in Italy, and almost the whole cities of Venice and Genoa, are built of very fine marble; and the perfection of the delicate carvings, however aged, is most remarkable. I remember a church, near Pavia, coated with the finest and most expensive marbles; a range of beautifully sculptured medallions running round its base, though old, were as distinct and fine in their execution as if they had just come out of the sculptor's studio. If, therefore, the gluten of the sandstone be either calcareous or siliceous, it will naturally produce the effect above alluded to, though it is certainly singular that the stone should be soft when first quarried. Sandstone is a rock in which you seldom see many cracks or fissures in the strata: they are generally continuous and solid. Now, there may be a certain degree of density in the mass, which could not be increased without producing, as in granite, fissures running through it: the particles may be supposed to be held in a certain degree of tension, and there may be a tendency to what the French call _assaissement_ (I do not know the English term), which is, nevertheless, resisted by the stone _en masse_; and a quantity of water may likewise be held, not in a state of chemical combination, but in one of close mixture with the rock. On being broken or quarried, the _assaissement_ may take place, the particles of stone may draw closer together, the attraction become stronger; and, on the exposure to the air, the water, however intimately combined, will, in a process of years, be driven off, occasioning the consolidation of the calcareous, and the near approach of the siliceous, particles, and a consequent gradual induration of the whole body of the stone. I offer this supposition with all diffidence; there may be many other causes, which cannot be developed until proper experiments have been made. It would be interesting to ascertain the relative hardness of different specimens of sandstone, taken from different depths in a bed, the surface of which was exposed to the air, as of specimens exposed to the air for different lengths of time. J. R. HERNE HILL, _July 25, 1836._
[Footnote 27: London's _Magazine of Natural History_, Vol. vii., pp. 644-5. The note was illustrated by engravings from two sketches by the author of the Aiguille de Servoz and of the Aiguille Dru, and by a diagram explanatory of its last sentence but one.--ED.] [Footnote 28: "A small neat copy of a sketch carefully taken on the spot," which, according to the editor of the magazine, accompanied this communication, was not, however, published. See the magazine.--ED.] [Footnote 29: Loudon's _Magazine of Natural History_, Vol. ix., No. 65, pp. 488-90.--ED.] [Footnote 30: The question here discussed was originally asked in the magazine (Vol. ix., pp. 379-80) by Mr. W. Perceval Hunter with reference to the condition of Bodiam Castle, in Sussex.--ED.] _ |
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