Theory of the Earth Volume Ii Part 14

Another example of the same kind, with regard to the bed of the Rhine, we have from the same author. (Discours, etc. page 259.)

_De Richenau a Coire, Troyen, et Saint-Gal._

Pour aller a Coire on pa.s.se le port qui est sur le haut Rhin; en cotoyant ce fleuve, qui coule dans un fond, on entre dans une plaine de niveau, qui n'a qu'une pente tres insensible de trois quarts de lieue; le fond du terrain n'est qu'un amas de pierres roulees de toutes especes. Les deux cotes sont bordes de montagnes calcaire qui courent parallelement entr'elles. Celle de la gauche, au pied de laquelle coule le Rhine, est tres rapide et perpendiculaire a son sommet; celle qui est a droite de la plaine ou pet.i.t vallon, puisqu'il se trouve entre des montagnes, est moins haute, plus boisee, et couverte de sapins. Le vallon est aussi couvert, en partie, de tres-grands et beaux pins; mais ce qu'on y voit de plus remarquable, c'est une douzaine de gros mamelons ou butes, elevees de cinquante a soixante toises, plus ou moins isolee, et a differentes distances les unes des autres; ces butes sont rondes, la plupart allongees dans le sens du vallon, et composees de debris calcaires et de sables; le fond du vallon est mele de plus d'especes de galets. On ne croit pas se tromper en disant que ce vallon a ete rempli de matieres apportees par les eaux jusqu'a la hauteur ou sont encore actuellement les mamelons; que de nouvelles inondations ont ensuite creuse et entraine ce qui manque de terrain a ces mamelons; que c'est en circulant autour de ces mamelons que les eaux leur ont donne la forme ronde; et surtout allongee dans le sens du vallon, et que c'est par le moyen de ces memes eaux que le fond actuel de cette plaine a pris ce niveau et cette pente insensible vers un pays plus ouvert qui est au-dela. On a deja fait mention de pareils mamelons qui se trouvent dans le vallon du Vallais parcouru par le Rhone.

These examples may also be supported by what this author observes in another place[25].

[Footnote 25: Discours, etc. page 201.]

Le vallon ou est situe Meiringen, est visiblement forme par le depot des eaux, il est de niveau, et s'etend trois lieues en longueur jusqu'au lac de Brientz, a la suite duquel est le meme terrain nivele, qui va jusqu'au lac de Thun, dont on a parle. Une autre observation qui concourt a favoriser ce sentiment, c'est que toutes les roches calcaires, qui entourent le vallon, sont a pic, qu'on y remarque des cavites circulaires et des enfoncemens a meme hauteur et a differents points, qui constatent la fouille et le mouvement des eaux contre ces parois.

Thus we have seen the operation of the atmospheric elements degrading mountains, and hollowing out the valleys of this earth.

The land which comes from the mineral region in a consolidated state, in order to endure the injuries of those atmospheric elements, must be resolved in time for the purposes of fertilising the surface of this earth. In no station whatever is it to be exempted from the wasting operations, which are equally necessary, in the system of this world, as were those by which it had been produced. But with what wisdom is that destroying power disposed! The summit of the mountain is degraded, and the materials of this part, which in a manner has become useless from its excessive height, are employed in order to extend the limits of the sh.o.r.e, and thus increase the useful basis of our dwellings. It is our business to trace this operation through all the intermediate steps of that progress, and thus to understand what we see upon the surface of this earth, by knowing the principles upon which the system of this world proceeds.

CHAP. XI.

_Facts and Opinions concerning the Natural Construction of Mountains and Valleys._

The valley of the Rhone is continued up to the mountain of St. Gothard, which may be considered as the centre of the Continent, since, from the different sides of this mountain, the water runs in all directions. To the German Sea it runs by the Rhine, to the Mediterranean by the Rhone, and to the Adriatic by the Po. Here it may be proper to take a general view of this mountainous country, or that great ma.s.s of rock or solid strata which has been either formed originally in its present shape, or has been excavated by the constant operation of water running from the summit in all the different directions.

On the one hand, it is supposed that the forming cause which had produced those mountains, in collecting their materials at the bottom of the sea, had also determined the shape in which their various ridges are at present found; on the other hand, it is supposed that the destructive causes, which operate in degrading mountains, have immediately contributed to produce their present forms, and that it is only mediately or more remotely that this shape has been determined by mineral operations and the const.i.tution of the solid parts, which thus oppose the wearing operations of the surface with different degrees of hardness and solidity. Whether natural appearances correspond with the one or the other of those two different suppositions, every person who has the opportunity of making such an examination, and has sufficient knowledge of the subject to judge from his observation, will determine for himself.

I will here give the opinion of a person who has had great opportunities for this purpose, who is an intelligent as well as an attentive observator, and who has had particularly this question in his view. It is from 'Tableaux de la Suisse'[26].

[Footnote 26: Discours sur l'Histoire Naturelle de la Suisse.]

Quand nous nous sommes trouve sur ces points eleves, nous avons toujours considere le total des montagnes prises ensemble, leurs situations respectives, les unes par rapport aux autres; afin de reconnoitre, s'il y avoit quelque chose de constant dans leurs position; rien n'est plus varie. Dans la grande chaine de montagnes qui separe le canton de Berne du Vallais d'un cote, et les Alpes qui separent le Vallais de la Savoie de l'autre, en considerant le course du Rhone sous differens points de vue, on n'a point vu que les angles saillans de ces tres hautes montagnes fussent opposes aux angles rentrans des montagnes qui sont vis-a-vis; Le fameux vallon qui est sur le haut du Saint-Gothard, le point le plus eleve de l'Europe, contredit egalement cette observation, aussi que les positions de la plus grande partie des montagnes qui forment son vaste circuit. Le vallon de Scholenen, qui a plus de huit lieues, et dans lequel la Reusse coule du sommet du Saint-Gothard jusqu'au lac de Lucerne, offre a peine quelques exemples d'angles rentrans opposes a des angles saillans. Les nombreux vallons que nous avons constamment traverses ceux qui conduisent au Grindelwald, et celui qui mene au pays de Hasli qui sont sous nos yeux, n'etablirent pas d'avantage cette correspondance d'angles saillans et angles rentrans, qu'on regarde comme si constante. Dans les montagnes ba.s.ses, du troisieme et quatrieme ordre, ou inferieures, on remarque plus souvent cette correspondance, encore n'est-elle pas constante: les eaux ordinaires ont forme ces vallons; mais si on veut donner une theorie generale, c'est a.s.surement dans les plus hautes montagnes qu'il faut prendre ses exemples. Ce qui se trouve au-dessous de ces points les plus eleves, a pris sa forme de la disposition meme des plus hauts sommets.

M. de Saussure, in his second volume of _Voyages dans les Alpes_, gives the strongest confirmation to the theory of the gradual degradation of mountains by the means of rain.

-- 920. Je reviens aux observations. Il en est une tres importante pour la theorie de la terre, dont on peut du haut du Cramont apprecier la valeur, mieux que d'aucun autre site; je veux parler de la fameuse observation de _Bourguet_ sur la correspondance des angles saillans avec les angles rentrans des vallees. J'ai a deja dit un mot dans le 1er.

volume, -- 577, mais j'ai renvoye a ce chapitre les developpemens que je vais donner.

Ce qui avoit fait regarder cette observation comme tres-importante, c'est que l'on avoit cru qu'elle pourroit servir a demontrer que les vallees ont ete creusees par des courans de la mer, dans le temps ou elle couvroit encore les montagnes; ou que les montagnes qui bordent ces vallees avoient ete elles-memes formees par l'acc.u.mulation des depots rejetes sur les bords de ces memes courans.

Mais l'inspection des vallees que l'on decouvre du haut du Cramont demontre pleinement le peu de solidite de ces deux suppositions. En effet, toutes les vallees que l'on decouvre du haut de cette cime sont fermees, au moins a l'une de leurs extremites et quelques-unes a leurs deux extremites, par des cols eleves, ou meme par des montagnes d'une tres-grande hauteur: toutes sont coupees a angles droits par d'autres vallees, et l'on voit enfin clairement que la plupart d'entr'elles ont ete creusees, non point dans la mer, mais, ou au moment de sa retraite, ou depuis sa retraite, par les eaux des neiges et des pluies.

On a d'abord sous ses yeux la grande vallee de l'Allee-Blanche, qui etant parallele a la direction general de cette partie des Alpes, est du nombre de celles que je nomme _longitudinales_; et l'on voit cette vallee barree a l'une de ses extremites par le Col de la Seigne et a l'autre par le Col Ferret. En se retournant du cote de l'Italie, on voit plusieurs vallees a-peu-pres paralleles a celle-la, comme celle de la Tuile, celle du Grand Saint Bernard, qui toutes aboutissent, par le haut, a quelque Col tres-eleve, et par le bas, a la Doire, ou elles viennent se jeter vis-a-vis de quelque montagne qui leur correspond de l'autre cote de cette vallee.

Si l'on considere ensuite cette meme vallee de la Doire, qui descend de Courmayeur a Yvree, on la verra barree par le Mont-Blanc et par la chaine centrale, qui la coupent a angles droits dans la partie superieure. On verra cette meme vallee s'ouvrir, dans un es.p.a.ce de sept ou huit lieues, deux ou trois inflexions tout-a-fait brusques; et on la verra enfin coupee a angles droits par une quant.i.te de vallees qui viennent y verser leurs eaux, et qui sont elles memes coupees par d'autres, dont elles recoivent aussi le tribut. Or quand on reflechit a la largeur et a l'etendue des courans de la mer, peut-on concevoir que ces sillons etroits, barres, qui se coupent en echiquier a de tres-pet.i.tes distances, aient pu etre creuses par de semblables courans.

L'observation de la correspondance des angles, fut-elle aussi universelle qu'elle l'est peu, ne prouveroit donc autre chose, sinon que les vallees sont nees de la fissure et de l'ecartement des montagnes, ou qu'elles ont ete creusees par les torrens et les rivieres qui y coulent actuellement. On voit un grand nombre de vallees naitre, comme je l'ai fait voir au Bon-Homme, -- 767, sur les flancs d'une montagne; on les voit s'elargir et s'approfondir a proportion des eaux qui y coulent; un ruisseau qui sort d'une glacier, ou qui sort d'une prairie, creuse un sillon, pet.i.t d'abord, mais qui s'agrandit successivement a mesure que ses eaux grossissent, par la reunion d'autres sources ou d'autres torrens.

Il n'est meme pas necessaire, pour se convaincre de la verite da ces faits, de gravir sur le Cramont. Il suffit de jeter les yeux sur la premiere carte que l'on trouvera sous la main, des Pyrenees, de l'Apennin, des Alpes, ou de quelqu'autre chaine de montagnes que ce puisse etre. On y verra toutes les vallees indiquees par le cours des rivieres; on verra ces rivieres et les vallees dans lesquelles elles coulent, aboutir par une de leurs extremites au sommet de quelque montagne ou de quelque col eleve. Les replis tortueux d'un grand fleuve, indiqueront une vallee princ.i.p.ale, dans laquelle des torrens ou des rivieres qui indiquent d'autres vallees moins considerables, viennent aboutir, sous des angles plus ou moins approchans de l'angle droit. Or ces rivieres qui viennent de droite et de gauche se jeter dans la vallee princ.i.p.ale, ne s'accordent pas pour se jeter par paires dans le meme point du fleuve; elles sont comme les branches d'un arbre qui s'implantent alternativement sur son tronc, et par consequent, chaque pet.i.te vallee se jette dans la vallee princ.i.p.ale vis-a-vis d'une montagne. Et de plus on verra aussi sur les cartes que meme les plus grandes vallees ont presque toutes des etranglemens qui forment des ecluses, des fourches, des defiles.

Je ne pretends cependant pas que l'erosion des eaux pluviales, des torrens et des rivieres, soit l'unique cause de la formation des vallees: le redress.e.m.e.nt des couches des montagnes nous force a en admettre une autre, dont je parlerai ailleurs; j'ai voulu seulement prouver, ici que la correspondance des angles, lorsqu'elle a lieu dans les vallees, ne prouve point que ces vallees soient l'ouvrage des courans de la mer.

The place to which M. de Saussure here remits us is where he afterwards, in describing the _Val d'Aoste_, makes the following observation.

(-- 960.) Au-dela de Nuz, les montagnes qui bordent au midi la vallee, et dont on voit d'ici tres-bien la structure, sont composees de grandes couches appliquees les unes contre les autres, et terminees par des cimes aigues, escarpees contre le midi, elles tournent ainsi le dos a la vallee, dont la direction est toujours a 10 degres de l'est par nord.

Celles de la gauche que nous cotoyons, et qui sont de nature schisteuse, tournent aussi le dos a la vallee en s'elevant contre le nord. Je crois pouvoir conclure de la, que cette vallee est une de celles dont la formation tient a celle des montagnes memes, et non point a l'erosion des courans de la mer ou des rivieres. Les vallees de ce genre, paroissent avoir ete formees par un affaiss.e.m.e.nt partiel des couches des montagnes, qui ont consenti, dans la direction qu'ont actuellement ces vallees.

Here I would beg leave to differ a little from this opinion of M. de Saussure, at least from the manner in which it is expressed; for perhaps at bottom our opinions upon this subject do not differ much.

M. de Saussure says that the formation of this valley depends upon the mountains themselves, and not upon the erosion of the rivers. I agree with our author, so far as the mountains may have here determined the shape and situation of the valley; but, so far as this valley was hollowed out of the solid ma.s.s of our earth, there cannot be the least doubt that the proper agent was the running water of the rivers. The question, therefore, comes to this, How far it is reasonable to conclude that this valley had been hollowed out of the solid ma.s.s. Now, according to the present theory, where the strata consolidated at the bottom of the sea are supposed to be erected into the place of land, we cannot suppose any valley formed by another agent than the running water upon the surface, although the parts which are first to be washed away, and those which are to remain longest, must be determined by a concurrence of various circ.u.mstances, among which this converging declivity of the strata in the bordering mountains, doubtless, must be enumerated.

With regard to any other theory which shall better explain the present shape of the surface of the earth, by giving a cause for the changed position of the strata originally horizontal, I cannot form a judgment, as I do not understand by what means strata, which were formed horizontally, should have been afterwards inclined, unless it be that of a power acting under those strata, and first erecting them in relation to the solid globe on which they rested.

Besides, in supposing this valley original, and not formed by the erosion of the rivers, What effect should we ascribe to the transport of all those materials of the Alps, which it is demonstrable must have travelled through this valley? Whether is it more reasonable to suppose, on the one hand, that the action and attrition of all the hard materials, running for millions of ages between those two mountains, had hollowed out that ma.s.s which originally intervened; or, on the other, that this valley had been originally formed in its present shape, while thousands of other valleys have been hollowed out of the solid ma.s.s?

But to put this question out of doubt, with regard to this very valley of the river _Doire_, M de Saussure has given us the following decisive fact, -- 881: Immediatement au-dessus de cette source, est un rocher qui repond si precis.e.m.e.nt a un autre rocher de la meme nature, situe de l'autre cote de la vallee de Courmayeur, qu'on ne sauroit douter qu'ils n'aient ete anciennement unis par une montagne intermediaire, detruite par les ravages du temps.

Now, to see how little the situation of the strata influences the shape of the valleys, I shall transcribe the two paragraphs immediately following that which has given occasion to the present discussion.

Un peu au-dela de Nux, la vallee cesse d'etre large et plane, comme elle etoit dans le environs de la cite; elle devient etroite et tres variee; la sterile et sauvage, ici couverte de vergers et de prairies arrosees par la Doire.

-- 961. Les couches des montagnes a notre gauche, qui depuis la cite avoient constamment couru a l'est et monte au nord, paroissent changer a un quart de lieue du village de Chambaise, qui est a une lieue et un quart de Nux. Elles montent d'abord au sud-est, et peu plus loin droit au sud, tandis que l'autre cote de la vallee elles paroissent monter a l'est.

In every mountain, and in every valley, the solid parts below have contributed in some manner to determine the shape of the surface of the earth; but in no place is the original shape of the earth, such as it had first appeared above the sea, to be found. Every part of the land is wasted; even the tops of the mountains, over which no floods of water run, are degraded. But this wasting operation, which affects the solid rock upon the summit of the mountain, operates slowly in some places, compared with that which may be observed in others. Now, it is in the valleys that this operation is so perceptible; and it is in the valley that there is such a quick succession of things as must strike the mind of any diligent observer; but this is the reason why we must conclude, that at least all the valleys are the operation of running water in the course of time. If this is granted, we have but to consider the mountains as formed by the hollowing out of the valleys, and the valleys as hollowed out by the attrition of hard materials coming from the mountains. Here is the explanation of the general appearance of mountain and valley, of hill and dale, of height and hollow; while each particular shape must have its dependence, consequently its explanation, upon some local circ.u.mstance.

But, besides the general conformation of mountains and valleys, there may be also, in the forms of mountains, certain characters depending upon the species of substances or rocks of which they are composed, and the general manner in which those ma.s.ses are wasted by the operations of the surface. Thus there is some character in the external appearance of a hill, a mountain, or a ridge of hills and mountains; but this appearance is generally attended with various circ.u.mstances, or is so complicated in its nature, as to be always difficult to read; and it is but seldom that it affords any very particular information; although, after knowing all the state and circ.u.mstances of the case, I have always found the appearances most intelligible, and strictly corresponding with the general principle of atmospheric influence acting upon the particular structure of the earth below.

M. de Saussure has given us an observation of this kind, in describing the mountains through which the Rhone has made its way out of the Alps, at the bottom of the Vallee.

--. 1061. Plus loin le village de _Juviana_ ou Envionne on voit des rochers qui ont une forme que je nomme _moutonnee_; car on est tente de donner des noms a des modifications qui n'en ont pas, et qui ont pourtant un caractere propre. Les montagnes que je designe par cette expression sont composees d'un a.s.semblage de tetes arrondies, couvertes quelquefois de bois, mais plus souvent d'herbes, ou tout au plus de brousailles. Ces rondeurs contigues et repetees forment en grand l'effet d'une toison bien fournie, ou de ces perruques que l'on nomme aussi _moutonnees_. Les montagnes qui se presentent sous cette forme, sont presque toujours de rochers primitives, ou au moins des steat.i.tes; car je n'ai jamais vu aucune montagne de pierre a chaux ou d'ardoise revetir cette apparence. Les signes qui peuvent donner quelque indice de la nature des montagnes, a de grandes distances et au travers des plantes qui le couvrent, sont en pet.i.t nombre, et meritent d'etre etudies et consacres par des termes propres.

When philosophers propose vague theories of the earth, theories which contain no principle for investigating either the general disorder of strata or the particular form of mountains, such theories can receive no confirmation from the examination of the earth, nor can they afford any rule by which the phenomena in question might be explained. This is not the case when a theory presents both the efficient and final cause of those disorders in bodies which had been originally formed regular, and which shows the use as well as means for the formation of our mountains.

Here ill.u.s.tration and confirmation of the theory may be found in the examination of nature; and natural appearances may receive that explanation which the generalization of a proper theory affords.

The particular forms of mountains depend upon the compound operation of two very different causes. One of these consists in those mineral operations by which the strata of the earth are consolidated and displaced, or disordered in the production of land above the sea; the other again consists in those meteorological operations by which this earth is rendered a habitable world. In the one operation, loose materials are united, for the purpose of resisting the dissolving powers which act upon the surface of the earth; in the other, consolidated ma.s.ses are again dissolved, for the purpose of serving vegetation and entertaining animal life. But, in fulfilling those purposes of a habitable earth, or serving that great end, the land above the level of the sea is wasted, and the materials are transported to the bottom of the ocean from whence that consolidated land had come. At present we only want to see the cause of those particular shapes which are found among the most elevated places of our earth, those places upon which the wasting powers of the surface act with greatest energy or force.

In explaining those appearances of degraded mountains variously shaped, the fact we are now to reason upon is this; first, that in the consolidated earth we find great inequality in the resisting powers of the various consolidated bodies, both from the different degrees of consolidation which had taken place among them, and the different degrees of solubility which is found in the consolidated substances; and, secondly, that we find great diversity in the size, form, and positions of those most durable bodies which, by resisting longer the effects of the wearing operations of the surface, must determine the shape of the remaining ma.s.s. Now so far as every particular shape upon the surface of this earth is found to correspond to the effect of those two causes, the theory which gave those principles must be confirmed in the examination of the earth; and so far as the theory is admitted to be just, we have principles for the explanation of every appearance of that kind, whether from the forming or destroying operations of this earth, there being no part upon the surface of this earth in which the effect of both those causes must not more or less appear.

But though the effects of those two causes be evident in the conformation of every mountainous region, it is not always easy to a.n.a.lyse those effects so as to see the efficient cause. Without sections of mountains their internal structure cannot be perceived, if the surface which we see be covered with soil as is generally the case. It is true, indeed, that the solid bodies often partially appear through that covering of soil, and so far discover to us what is to be found within; but as those solid parts are often in disorder, we cannot, from a small portion, always judge of the generality. Besides, the solid parts of mountains is often a compound thing, composed both of stratified and injected bodies; it is therefore most precarious, from a portion which is seen, to form a judgment of a whole ma.s.s which is unexplored. Nevertheless, knowing the principles observed by nature both in the construction and degradation of mountains, and cautiously inferring nothing farther than the data will admit of, some conclusion may be formed, in reasoning from what is known to what is still unknown.

It is with this view that we are now to consider the general forms of mountains, such as they appear to us at a certain distance, when we have not the opportunity of examining them in a more perfect manner. For, though we may not thus learn always to understand that which is thus examined, we shall learn, what is still more interesting, viz. that those mountains have been formed in the natural operations of the earth, and according to physical rules that may be investigated.

We are to distinguish mountains as being either on the one hand soft and smooth, or on the other hand as hard and rocky. If we can understand those two great divisions by themselves, we shall find it easy to explain the more complex cases, where these two general appearances partially prevail. Let us therefore examine this general division which we have made with regard to the external character of mountains.

The soft and smooth mountains are generally formed of the schisti, when there is any considerable extent of such alpine or mountainous region. The substance is sufficiently durable to form a mountain, or sufficiently strong, in its natural state, to resist the greatest torrent of water; at the same time this fissible substance generally decays so completely, when exposed to the atmosphere, as to leave no salient rock exposed by which to characterise the mountain.

Of this kind are the schisti of Wales, of c.u.mberland, of the isle of Man, and of the south of Scotland. I do not say absolutely, that there is no other kind of material, besides the schisti which gives this species of mountain, but only that this is generally the case in alpine situations. It may be also formed of any other substance which has solidity enough to remain in the form of mountains, and at same time not enough to form salient rocks. Such, for example, is the chalk hills of the Isle of Wight and south of England. But these are generally hills of an inferior height compared with our alpine schisti, and hardly deserve the term of mountain.

This material of our smooth green mountains may be termed an argillaceous schistus; it has generally calcareous veins, and is often fibrous in its structure resembling wood, instead of being slatey, which it is in general. There is however another species of schistus, forming also the same sort of mountain; it is the micaceous quartzy schistus of the north of Scotland. Now it must be evident that the character of those mountains arises from there being no part of those schisti that resists the influence of the atmosphere, in exfoliating and breaking into soil; and this soil is doubtless of different qualities, according to the nature of those schisti from which the soil is formed.