Notes On Natural Science - Stratified Alps Of Savoy. 1863

________________________________________________
_ ON THE FORMS OF THE STRATIFIED ALPS OF SAVOY.[35]

290. The purpose of the discourse was to trace some of the influences which have produced the present external forms of the stratified mountains of Savoy, and the probable extent and results of the future operation of such influences.

The subject was arranged under three heads:--

I. The Materials of the Savoy Alps.
II. The Mode of their Formation.
III. The Mode of their subsequent Sculpture.

291. I. _Their Materials._--The investigation was limited to those Alps which consist, in whole or in part, either of Jura limestone, of Neocomian beds, or of the Hippurite limestone, and include no important masses of other formations. All these rocks are marine deposits; and the first question to be considered with respect to the development of mountains out of them is the kind of change they must undergo in being dried. Whether prolonged through vast periods of time, or hastened by heat and pressure, the drying and solidification of such rocks involved their contraction, and usually, in consequence, their being traversed throughout by minute fissures. Under certain conditions of pressure, these fissures take the aspect of slaty cleavage; under others, they become irregular cracks, dividing all the substance of the stone. If these are not filled, the rock would become a mere heap of debris, and be incapable of establishing itself in any bold form. This is provided against by a metamorphic action, which either arranges the particles of the rock, throughout, in new and more crystalline conditions, or else causes some of them to separate from the rest, to traverse the body of the rock, and arrange themselves in its fissures; thus forming a cement, usually of finer and purer substance than the rest of the stone. In either case the action tends continually to the purification and segregation of the elements of the stone. The energy of such action depends on accidental circumstances: first, on the attractions of the component elements among themselves; secondly, on every change of external temperature and relation. So that mountains are at different periods in different stages of health (so to call it) or disease. We have mountains of a languid temperament, mountains with checked circulations, mountains in nervous fevers, mountains in atrophy and decline.

292. This change in the structure of existing rocks is traceable through continuous gradations, so that a black mud or calcareous slime is imperceptibly modified into a magnificently hard and crystalline substance, inclosing nests of beryl, topaz, and sapphire, and veined with gold. But it cannot be determined how far, or in what localities, these changes are yet arrested; in the plurality of instances they are evidently yet in progress. It appears rational to suppose that as each rock approaches to its perfect type the change becomes slower; its perfection being continually neared, but never reached; its change being liable also to interruption or reversal by new geological phenomena. In the process of this change, rocks expand or contract; and, in portions, their multitudinous fissures give them a ductility or viscosity like that of glacier-ice on a larger scale. So that many formations are best to be conceived as glaciers, or frozen fields of crag, whose depth is to be measured in miles instead of fathoms, whose crevasses are filled with solvent flame, with vapor, with gelatinous flint, or with crystallizing elements of mingled natures; the whole mass changing its dimensions and flowing into new channels, though by gradations which cannot be measured, and in periods of time of which human life forms no appreciable unit.

293. II. _Formation._--Mountains are to be arranged, with respect to their structure, under two great classes--those which are cut out of the beds of which they are composed, and those which are formed by the convolution or contortion of the beds themselves. The Savoy mountains are chiefly of this latter class. When stratified formations are contorted, it is usually either by pressure from below, which raises one part of the formation above the rest, or by lateral pressure, which reduces the whole formation into a series of waves. The ascending pressure may be limited in its sphere of operation; the lateral one necessarily affects extensive tracts of country, and the eminences it produces vanish only by degrees, like the waves left in the wake of a ship. The Savoy mountains have undergone both these kinds of violence in very complex modes and at different periods, so that it becomes almost impossible to trace separately and completely the operation of any given force at a given point.

294. The speaker's intention was to have analyzed, as far as possible, the action of the forming forces in one wave of simple elevation, the Mont Saleve, and in another of lateral compression, the Mont Brezon: but the investigation of the Mont Saleve had presented unexpected difficulty. Its facade had been always considered to be formed by vertical beds, raised into that position during the tertiary periods; the speaker's investigations had, on the contrary, led him to conclude that the appearance of vertical beds was owing to a peculiarly sharp and distinct cleavage, at right angles with the beds, but nearly parallel to their strike, elsewhere similarly manifested in the Jurassic series of Savoy, and showing itself on the fronts of most of the precipices formed of that rock. The attention of geologists was invited to the determination of this question.

The compressed wave of the Brezon, more complex in arrangement, was more clearly defined. A section of it was given, showing the reversed position of the Hippurite limestone in the summit and lower precipices. This limestone wave was shown to be one of a great series, running parallel with the Alps, and constituting an undulatory district, chiefly composed of chalk beds, separated from the higher limestone district of the Jura and Lias by a long trench or moat, filled with members of the tertiary series--chiefly nummulite limestones and flysch. This trench might be followed from Faverges, at the head of the lake of Annecy, across Savoy. It separated Mont Vergi from the Mont Dorons, and the Dent d'Oche from the Dent du Midi; then entered Switzerland, separating the Moleson from the Diablerets; passed on through the districts of Thun and Brientz, and, dividing itself into two, caused the zigzagged form of the lake of Lucerne. The principal branch then passed between the high Sentis and the Glarnisch, and broke into confusion in the Tyrol. On the north side of this trench the chalk beds were often vertical, or cast into repeated folds, of which the escarpments were mostly turned away from the Alps; but on the south side of the trench, the Jurassic, Triassic, and Carboniferous beds, though much distorted, showed a prevailing tendency to lean towards the Alps, and turn their escarpments to the central chain.

295. Both these systems of mountains are intersected by transverse valleys, owing their origin, in the first instance, to a series of transverse curvilinear fractures, which affect the forms even of every minor ridge, and produce its principal ravines and boldest rocks, even where no distinctly excavated valleys exist. Thus, the Mont Vergi and the Aiguilles of Salouvre are only fragmentary remains of a range of horizontal beds, once continuous, but broken by this transverse system of curvilinear cleavage, and worn or weathered into separate summits.

The means of this ultimate sculpture or weathering were lastly to be considered.

* * * * *

296. III. _Sculpture._--The final reductions of mountainform are owing either to disintegration, or to the action of water, in the condition of rain, rivers, or ice, aided by frost and other circumstances of temperature and atmosphere.

All important existing forms are owing to disintegration, or the action of water. That of ice had been curiously over-rated. As an instrument of sculpture, ice is much less powerful than water; the apparently energetic effects of it being merely the exponents of disintegration. A glacier did not produce its moraine, but sustained and exposed the fragments which fell on its surface, pulverizing these by keeping them in motion, but producing very unimportant effects on the rock below; the roundings and striation produced by ice were superficial; while a torrent penetrated into every angle and cranny, undermining and wearing continually, and carrying stones, at the lowest estimate, six hundred thousand times as fast as the glacier. Had the quantity of rain which has fallen on Mont Blanc in the form of snow (and descended in the ravines as ice) fallen as rain, and descended in torrents, the ravines would have been much deeper than they are now, and the glacier may so far be considered as exercising a protective influence. But its power of carriage is unlimited, and when masses of earth or rock are once loosened, the glacier carries them away, and exposes fresh surfaces. Generally, the work of water and ice is in mountain surgery like that of lancet and sponge--one for incision, the other for ablution. No excavation by ice was possible on a large scale, any more than by a stream of honey; and its various actions, with their limitations, were only to be understood by keeping always clearly in view the great law of its motion as a viscous substance, determined by Professor James Forbes.

297. The existing forms of the Alps are, therefore, traceable chiefly to denudation as they rose from the sea, followed by more or less violent aqueous action, partly arrested during the glacial periods, while the produced diluvium was carried away into the valley of the Rhine or into the North Sea. One very important result of denudation had not yet been sufficiently regarded; namely, that when portions of a thick bed (as the Rudisten-kalk) had been entirely removed, the weight of the remaining masses, pressing unequally on the inferior beds, would, when these were soft (as the Neocomian marls), press them up into arched conditions, like those of the floors of coal-mines in what the miners called "creeps." Many anomalous positions of the beds of Spatangenkalk in the district of the Lake of Annecy were in all probability owing to this cause: they might be studied advantageously in the sloping base of the great Rochers de Lanfon, which, disintegrating in curved, nearly vertical flakes, each a thousand feet in height, were nevertheless a mere outlying remnant of the great horizontal formation of the Parmelan, and formed, like it, of very thin horizontal beds of Rudisten-kalk, imposed on shaly masses of Neocomian, modified by their pressure. More complex forms of harder rock were wrought by the streams and rains into fantastic outlines; and the transverse gorges were cut deep where they had been first traced by fault or distortion. The analysis of this aqueous action would alone require a series of discourses; but the sum of the facts was that the best and most interesting portions of the mountains were just those which were finally left, the centers and joints, as it were, of the Alpine anatomy. Immeasurable periods of time would be required to wear these away; and to all appearances, during the process of their destruction, others were rising to take their place, and forms of perhaps far more nobly organized mountain would witness the collateral progress of humanity.

J. R.

FOOTNOTES:

[Footnote 35: Read by Mr. Ruskin at the weekly evening meeting of the Royal Institution (see _Proceedings_, vol. iv., pp. 142-46), June 5, 1863.--ED.] _

Read next: Notes On Natural Science: Intellectual Conception And Animated Life. 1871

Read previous: Notes On Natural Science: Tree Twigs. 1861

Table of content of On the Old Road Volume 2 (of 2)


GO TO TOP OF SCREEN

Post your review
Your review will be placed after the table of content of this book