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Theory of the Earth, by James Hutton, [1788 and 1795], at sacred-texts.com
Facts and Opinions concerning the Natural
Construction of Mountains and Valleys.
The valley of the Rhône 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 mass 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 constitution 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.
«Quand nous nous sommes trouvé sur ces points élevés, nous avons toujours considéré le total des montagnes prises ensemble, leurs situations respectives, les unes par rapport aux autres; afin de reconnoître, s'il y avoit quelque chose de constant dans leurs position; rien n'est plus varié. Dans la grande chaîne de montagnes qui sépare le canton de Berne du Vallais d'un côté, et les Alpes qui séparent le Vallais de la Savoie de l'autre, en considérant le course du Rhône sous differens points de vue, on n'a point vu que les angles saillans de ces très hautes montagnes fussent opposés aux angles rentrans des montagnes qui sont vis-à-vis; Le fameux vallon qui est sur le haut du Saint-Gothard, le point le plus élevé de l'Europe, contredit également 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 à peine quelques exemples d'angles rentrans opposés à des angles saillans. Les nombreux vallons que nous avons constamment traversés ceux qui conduisent au Grindelwald, et celui qui mène au pays de Hasli qui sont sous nos yeux, n'établirent pas d'avantage cette correspondance d'angles saillans et angles rentrans, qu'on regarde comme si constante. Dans les montagnes basses, du troisième et quatrième ordre, ou inférieures, on remarque plus souvent cette correspondance, encore n'est-elle pas constante: les eaux ordinaires ont formé ces vallons; mais si on veut donner une théorie générale, c'est assurement dans les plus hautes montagnes qu'il faut prendre ses exemples. Ce qui se trouve au-dessous de ces points les plus élevés, a pris sa forme de la disposition même 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 très importante pour la théorie de la terre, dont on peut du haut du Cramont apprécier 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 vallées. J'ai a déjà dit un mot dans le 1er. volume, § 577, mais j'ai renvoyé à ce chapitre les developpemens que je vais donner.
«Ce qui avoit fait regarder cette observation comme très-importante, c'est que l'on avoit cru qu'elle pourroit servir à démontrer que les vallées ont été creusées par des courans de la mer, dans le temps où elle couvroit encore les montagnes; ou que les montagnes qui bordent ces vallées avoient été elles-mêmes formées par l'accumulation des dépôts rejetés sur les bords de ces mêmes courans.
«Mais l'inspection des vallées que l'on découvre du haut du Cramont démontre pleinement le peu de solidité de ces deux suppositions. En effet, toutes les vallées que l'on découvre du haut de cette cime sont fermées, au moins à l'une de leurs extrémités et quelques-unes à leurs deux extrémités, par des cols élevés, ou même par des montagnes d'une très-grande hauteur: toutes sont coupées à angles droits par d'autres vallées, et l'on voit enfin clairement que la plupart d'entr'elles ont été creusées, 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 vallée de l'Allée-Blanche, qui étant parallèle à la direction général de cette partie des Alpes, est du nombre de celles que je nomme longitudinales; et l'on voit cette vallée barrée à l'une de ses extrémités par le Col de la Seigne et à l'autre par le Col Ferret. En se retournant du côté de l'Italie, on voit plusieurs vallées à-peu-près parallèles à celle-là, comme celle de la Tuile, celle du Grand Saint Bernard, qui toutes aboutissent, par le haut, à quelque Col très-élevé, et par le bas, à la Doire, où elles viennent se jeter vis-à-vis de quelque montagne qui leur correspond de l'autre côté de cette vallée.
«Si l'on considère ensuite cette même vallée de la Doire, qui descend de Courmayeur à Yvrée, on la verra barrée par le Mont-Blanc et par la chaîne centrale, qui la coupent à angles droits dans la partie supérieure. On verra cette même vallée s'ouvrir, dans un espace de sept ou huit lieues, deux ou trois inflexions tout-à-fait brusques; et on la verra enfin coupée à angles droits par une quantité de vallées qui viennent y verser leurs eaux, et qui sont elles mêmes coupées par d'autres, dont elles reçoivent aussi le tribut. Or quand on réfléchit à la largeur et à l'étendue des courans de la mer, peut-on concevoir que ces sillons étroits, barrés, qui se coupent en échiquier à de très-petites distances, aient pu être creusés 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 vallées sont nées de la fissure et de l'écartement des montagnes, ou qu'elles ont été creusées par les torrens et les rivières qui y coulent actuellement. On voit un grand nombre de vallées naître, comme je l'ai fait voir au Bon-Homme, § 767, sur les flancs d'une montagne; on les voit s'élargir 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, petit d'abord, mais qui s'agrandit successivement à mesure que ses eaux grossissent, par la réunion d'autres sources ou d'autres torrens.
«Il n'est même pas nécessaire, pour se convaincre de la vérité 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 Pyrénées, de l'Apennin, des Alpes, ou de quelqu'autre chaîne de montagnes que ce puisse être. On y verra toutes les vallées indiquées par le cours des rivieres; on verra ces rivieres et les vallées dans lesquelles elles coulent, aboutir par une de leurs extrémités au sommet de quelque montagne ou de quelque col élevé. Les replis tortueux d'un grand fleuve, indiqueront une vallée principale, dans laquelle des torrens ou des rivieres qui indiquent d'autres vallées moins considérables, 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 vallée principale, ne s'accordent pas pour se jeter par paires dans le même point du fleuve; elles sont comme les branches d'un arbre qui s'implantent alternativement sur son tronc, et par conséquent, chaque petite vallée se jette dans la vallée principale vis-à-vis d'une montagne. Et de plus on verra aussi sur les cartes que même les plus grandes vallées ont presque toutes des étranglemens qui forment des écluses, des fourches, des défilés.
«Je ne prétends cependant pas que l'érosion des eaux pluviales, des torrens et des rivieres, soit l'unique cause de la formation des vallées: le redressement des couches des montagnes nous force à 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 vallées, ne prouve point que ces vallées 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 vallée, et dont on voit d'ici très-bien la structure, sont composées de grandes couches appliquées les unes contre les autres, et terminées par des cimes aigues, escarpées contre le midi, elles tournent ainsi le dos à la vallée, dont la direction est toujours à 10 degrés de l'est par nord. Celles de la gauche que nous côtoyons, et qui sont de nature schisteuse, tournent aussi le dos à la vallée en s'élevant contre le nord. Je crois pouvoir conclure de là, que cette vallée est une de celles dont la formation tient à celle des montagnes mêmes, et non point à l'érosion des courans de la mer ou des rivières. Les vallées de ce genre, paroissent avoir été formées par un affaissement partiel des couches des montagnes, qui ont consenti, dans la direction qu'ont actuellement ces vallées.»
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 mass 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 mass. 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 circumstances, 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 mass 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 mass?
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: «Immédiatement au-dessus de cette source, est un rocher qui répond si précisément à un autre rocher de la même nature, situé de l'autre côté de la vallée de Courmayeur, qu'on ne sauroit douter qu'ils n'aient été anciennement unis par une montagne intermédiaire, détruite 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 vallée cesse d'être large et plane, comme elle étoit dans le environs de la cité; elle devient étroite et très variée; là stérile et sauvage, ici couverte de vergers et de prairies arrosées par la Doire.
«§ 961. Les couches des montagnes à notre gauche, qui depuis la cité avoient constamment couru à l'est et monté au nord, paroissent changer à un quart de lieue du village de Chambaise, qui est à 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 côté de la vallée elles paroissent monter à 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 circumstance.
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 masses 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 circumstances, 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 circumstances 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 Rhône has made its way out of the Alps, at the bottom of the Vallée.
«§. 1061. Plus loin le village de Juviana ou Envionne on voit des rochers qui ont une forme que je nomme moutonnée; car on est tenté de donner des noms à des modifications qui n'en ont pas, et qui ont pourtant un caractère propre. Les montagnes que je désigne par cette expression sont composées d'un assemblage de têtes arrondies, couvertes quelquefois de bois, mais plus souvent d'herbes, ou tout au plus de brousailles. Ces rondeurs contiguës et répétées forment en grand l'effet d'une toison bien fournie, ou de ces perruques que l'on nommé aussi moutonnées. Les montagnes qui se présentent sous cette forme, sont presque toujours de rochers primitives, ou au moins des stéatites; car je n'ai jamais vu aucune montagne de pierre à chaux ou d'ardoise revêtir cette apparence. Les signes qui peuvent donner quelque indice de la nature des montagnes, à de grandes distances et au travers des plantes qui le couvrent, sont en petit nombre, et méritent d'être étudiés et consacrés 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 illustration 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 masses 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 mass. 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 analyse 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 mass 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 Cumberland, 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.
Such mountains are necessarily composed of rounded masses, and not formed of angular shapes. They are covered with soil, which is more or less either stoney or tender, sterile or fertile, according to the materials which produce that soil. The fertile mountains are green and covered with grass; the sterile mountains again are black, or covered with heath in our climates.
Thus we have a general character of smooth and rounded mountains; and also a distinction in that general character from the produce of the soil indicating the nature of the solid materials, as containing, either on the one hand calcareous and argillaceous substances, or, on the other, as only containing those that are micaceous and siliceous.
With regard again to the other species of mountain, which we have termed rocky, we must make a subdistinction of those which are regular, and those in which there is no regularity to be perceived. It must be plain that it is only of those which have regularity that we can form a theory. It is this, that the regularity in the shape of those mountains arises from the rock of the mountain being either on the one hand an uniform solid mass, or on the other hand a stratified mass, or one formed upon some regular principle distinguishable in the shape. In the first of these, we have a conical or pyramidal shape, arising from the gradual decay of the rock exposed to the destructive causes of the surface, as already explained in this chapter. In the second, again, we find the original structure of the mass influencing the present shape in conjunction with the destructive causes, by which a certain regularity may be observed. Now, this original shape is no other than that of beds or strata of solid resisting rock, which may be regularly disposed in a mountain, either horizontally, vertically, or in an inclined position; and those solid beds may then affect the shape of the mountain in some regular or distinguishable manner, besides the other parts of its shape which it acquires upon the principle of decay.
In distinguishing, at a distance, those regular causes in the form of mountains, we may not be able to tell, with certainty, what the substance is of which the mountain is composed; yet, with regard to the internal structure of that part of the earth, a person of knowledge and experience in the subject may form a judgment in which, for coming at truth, there is more than accident; there is even often more than probable conjecture. Thus, a horizontal bed of rock forms a table mountain, or such as M. Bouguer found in the valley of the Madelena. An inclined rock of this kind forms a mountain sloping on the one side, and having a precipice upon the upper part of the other side, with a slope of fallen earth at the bottom; such as the ridges observed by M. de Saussure from the top of the Cramont, having precipices upon one side, which also had a respect to certain central points, an observation which draws to more than the simple structure of the mountain. Were it vertical, again, it would form a rocky ridge extended in length, and having its sides equally sloped, so far as the other circumstances of the place would permit.
Therefore, whether we suppose the mountains formed of a rock in mass, or in that of regular beds, this must have an influence in the form of this decaying surface of the earth, and may be distinguished in the shape of mountains. It is but rarely that we find mountains formed altogether of rock, although we often find them of the other sort, where little or nothing of rock is to be seen. But often also we find the two cases variously compounded. This is the source of the difficulty which occurs in the reading of the external characters of mountains; and this is one of the causes of irregularity in the form of mountains, by which there is always some degree of uncertainty in our judgment from external appearances.
We may form another distinction with regard to the structure of mountains, a distinction which depends upon a particular cause, and which will afford an explanation of some other appearance in the surface of the earth.
Mountains in general may be considered as, being either on the one hand associated, or on the other insulated; and this forms a distinction which may be explained in the theory, and afford some ground for judging of the internal structure from the external appearance.
The associated mountains are formed by the wearing down of the most decayable, or softer places, by the collected waters of the surface; consequently there is a certain similarity, or analogy, of the mountains formed of the same materials, and thus associated. The highest of those mountains should be near the center of the mass; but, in extensive masses of this kind, there may also be more than one center. Nor are all the associated mountains to be of one kind, however, to a certain extent, similarity may be expected to prevail among them.
It must now be evident, that when we find mountains composed of very different materials, such as, e.g. of granite, and of lime-stone or marl, and when the shape of those mountains are similar, or formed upon the same principle, such as, e.g. the pyramidal mountains of the Alps, we are then to conclude, as has already been exemplified (chap. 9. page 306.) that those consolidated masses of this earth had been formed into the pyramidal mountains in the same manner. We have there also shown that this principle of formation is no other than the gradual decay of the solid mass by gravity and the atmospheric influences. Consequently, those pyramidal mountains, though composed of such different materials, may, at a certain distance, where smaller characteristic distinctions may not be perceivable, appear to be of the same kind; and this indeed they truly are, so far as having their general shape formed upon the same principle.
We come now to treat of insulated mountains. Here volcanos must be mentioned as a cause. By means of a volcano, a mountain may be raised in a plain, and a volcanic mountain might even rise out of the sea. The formation of this species of mountain requires not the wearing operations of the earth which we have been considering as the modifier of our alpine regions. This volcanic mountain has a conical shape, perhaps more from the manner of its formation which is accretion, than from the wasting of the surface of the earth. It is not, however, of this particular specie of mountain that I mean to treat, having had no opportunity of examining any of that species.
The genus of mountain which we are now considering, is that of the eruptive kind. But there is much of this eruptive matter in the bowels of the earth, which, so far as we know, never has produced a volcano. It is to this species of eruption that I am now to attribute the formation of many insulated mountains, which rise in what may be termed low countries, in opposition to the highlands or alpine situations. Such is Wrekin in Shropshire, which some people have supposed to have been a volcano. Such are the hundred little mountains in the lowlands of this country of Scotland, where those insulated hills are often called by the general term Law; as, for example, North Berwick Law.
When masses of fluid matter are erupted in the mineral regions among strata which are to form our land; and when those elevated strata are, in the course of time, wasted and washed away, the solid mass of those erupted substances, being more durable than the surrounding strata, stand up as eminences in our land. Now these often, almost always, form the small insulated mountains which are found so frequently breaking out in the lowlands of Scotland. They appear in various shapes as well as sizes; and they hold their particular form from the joint operation of two different causes; one is the extent and casual shape of the erupted mass; the other is the degradation of that mass, which is wasted by the influences of the atmosphere, though wasted slower than the strata with which it was involved.
When the formation of this erupted mass has been determined by the place in any regular form, which may be distinguished in the shape of a mountain, it gives a certain character which is often not difficult to read. Thus, our whin-stone, interjected in flat beds between the regular strata, often presents its edge upon, or near the summit of our insolated mountains and eminences. They are commonly in the form of inclined planes; and, to a person a little conversant in this subject, they are extremely distinguishable in the external form of the hill.
We have a good example of this in the little mountain of Arthur's Seat, by this town of Edinburgh. This is a peaked hill of an irregular erupted mass; but on the south and north sides of this central mass, the basaltic matters had been forced also in those inclined beds among the regular strata. On the north side we find remarkable masses of whin-stone in that regular form among the strata, and lying parallel with them. The most conspicuous of these basaltic beds forms the summit of the hill which is called Salisbury Craig. Here the bed of whin-stone, more than 60 or 80 feet thick, rises to the west at an angle of about 40 degrees; it forms the precipicious summit which looks to the west; and this is an appearance which is distinguishable upon a hundred other occasions in the hills and mountains of this country.
Rivers make sections of mountains through which they pass. Therefore, nothing is more interesting for bringing to our knowledge the former state of things upon the surface of this earth, than the examination of those valleys which the rivers have formed by wearing down the solid parts of alpine countries. We have already seen that the wide extensive valley of the Rhône, between Loiche and Kolebesche, as well as the whole extensive circus of the Rosa mountains, has on each side mountains of the same substances, the strata of which are horizontal; consequently, here the valley must have been hollowed out of the solid rock; for there is no natural operation by which those opposite mountains of horizontal strata could have been formed, except in the continuation of those beds. We are therefore to conclude, that the solid strata between those ridges of lofty mountains had been continuous.
The most perfect confirmation which this theory could receive, would be to find that those ridges of mountains, which the Rhône divides in issuing from the Alps into the plain, had been also united, in forming one continued mass of solid rocks. But the observations of M. de Saussure, who has most carefully examined this subject, will leave no room to doubt of that fact.
This view of the entry to the valley of the Rhône is too interesting not to give it here a place. It follows immediately after that which we have last transcribed.
«Ces montagnes que j'allai sonder au haut des prairies qui les séparent de la grande route, sont composées d'un mélange très ressemblant au précédent, et ce sont-là, les derniers rochers primitifs que l'on rencontre en sortant des Alpes par cette vallée. Le village de Juviana, dont ils occupent les derrières, est encore à une lieue de St Maurice.
«§ 1062. A l'extrémité de ces rochers, on voit une grande ravine, ou plutôt une vallée ouverte du nord au midi, dans laquelle coule le torrent de St. Barthelemi. Cette vallée termine les montagnes primitives que je viens de décrire: au-delà commencent les montagnes calcaires. Cependant le pied de la montagne primitive, coupé par le torrent, est demeuré engagé sous les premières couches de la montagne calcaire.
«Au travers de cette vallée, on voit de hautes montagnes couverte de neige, situées derriere celles qui bordent notre route. La plus haute et la plus remarquable de ces montagnes se nomme la Dent ou l'Aiguille du Midi. De l'autre côté du Rhône, on voit une autre cime aussi très-élevée, qui se nomme la Dent ou l'Aiguille de la Morele. Ces deux hautes cimes ont entr'elles une correspondance de hauteur, de forme, et même de matière tout-à-fait singulière. L'une et l'autre présentent leurs escarpemens à la vallée du Rhône. Leurs cimes crénelées sont de la même couleur brune. Sous ces cimes brunes, on voit de part et d'autre une bande grise, qui paroît horizontale, et au-dessous de cette bande grise, le rocher, dans l'une comme dans l'autre, reprend sa couleur jaunâtre. Ces montagnes sont sûrement secondaires, les bandes grises paroissent être de pierre à chaux, et les jaunes de schiste argilleux et de grès, à en juger du moins à cette distance, car je ne les ai point observées de plus près. Elles paroissent aussi appartenir à des chaînes secondaires qui passent derrière les chaînes primitives, que nous avons observées sur les bords du Rhône, et quoique les bandes jaunes et grises que l'on y observe, semblent horizontales, je ne doute point que les couches mêmes, dont ces bandes sont les sections, ne descendent en arriere avec assez de rapidité; le escarpemens de ces montagnes en font une preuve à-peu-près certaine.
«Ces hautes montagnes auroient-elles été anciennement liées entr'elles par des intermédiaires de la même nature, que couvroient, et les primitives que nous avons observées, et toute cette vallée dans laquelle coule aujourd'hui le Rhône? Je me garderois bien de l'affirmer, mais je ferois tenté de le croire.
«§ 1063. Depuis la vallée dont je viens de parler, et qui termine au couchant les montagnes primitives, celles qui suivent jusques à St. Maurice, sont de nature calcaire à couches épaisses et suivies. Ces couches s'élèvent contre les primitives que nous avons côtoyées; et celles qui en sont les plus voisines paroissent fort tourmentées; ici fléchies, là rompues. Après une interruption, ces rochers sont suivis d'autres rochers, aussi calcaires, coupés à pic du côté de la vallée, et composés de grandes assises horizontales. Ces rochers forment une enceinte demi-circulaire, qui vient presque se joindre à ceux qui bordent la rive droite du Rhône, et former ainsi l'entrée de cette vallée, dont le fleuve ne sort que par une issue très-étroite.
«La ville de St. Maurice est ainsi renfermée par cette enceinte de rochers, dont les bancs épais, bien suivis, séparés par des cordons de verdure, et couronnés par des forêts, avec un hermitage niché entre ces bancs, présente une aspect singulier et pittoresque.
«§ 1064. Les rochers correspondans de l'autre côté du Rhône, ou sur la rive droite de ce fleuve sont aussi calcaires. La montagne qui domine cette rive, un peu au-dessus de St. Maurice, est composée de couches contournées, froissées et repliées de la maniere la plus étrange. Ce qu'il y a encore de remarquable, c'est que ces couches ainsi repliées en ont d'autres à côté d'elles qui sont planes, presque verticales, et d'autres sous elles, qui sont horizontales. Il faudroit avoir observé de près ce singulier rocher, et avoir déterminé comment et jusqu'à quel point ces couches sont unies entr'elles pour former les conjectures sur leur origine. Car la vallée est trop large pour que l'on puisse en juger avec précision d'une rive à l'autre.
«On voit avec peine que cette large vallée soit aussi peu cultivée; elle est presque partout couverte, ou de marais, ou de débris des montagnes voisines.
«§ 1065. Avant de quitter cette vallée, je jetterai un coup-d'oeil général sur la singulière suite de rochers qui composent la chaîne que nous venons d'observer.
«Les deux extrémités sont calcaires, avec cette différence, que celle qui est la plus près de Martigny est mêlée de mica, tandis que celle de St Maurice n'en contient point. Entre ces calcaires sont refermées des rochers que l'on regarde comme primitives; et au milieu de ces roches on trouve des ardoises et des poudingues. On fait que ce dernier genre est ordinairement classé parmi les montagnes tertiaires, ou de la formation la plus récente. Mais ces poudingues-ci, qui ne contiennent aucun fragment de pierre calcaire, qui ne sont même point unis par un gluten calcaire, ne sont vraisemblablement pas postérieures à la formation des montagnes calcaires, ou du moins ils ne doivent point être confondus avec ces grès et ces poudingues de formation nouvelle, qui entrent dans la composition des montagnes du troisième ordre.
«Quant aux ardoises que se trouvent interposées au milieu de ces grès et de ces poudingues, § 1054, elles sont de nature argilleuse, et dans l'ordre des pierres que l'on nomme secondaires.
«Ces ardoises, de même que toutes les pierres de ces montagnes, ont leurs couches dans une situation verticale: mais nous avons vu qu'il y a lieu de croire qu'elles ont été anciennement horizontales.»
It is singularly fortunate that such remarkable appearances, as are found in the rocks of this place, had called the attention of M. de Saussure to investigate a subject so interesting to the present theory; and it is upon this, as well as on many other occasions, that the value of those observations of natural history will appear. They are made by a person eminent for knowledge; and they are recorded with an accuracy and precision which leaves nothing more to be desired.
From Martigny to St. Maurice, about three leagues, there is a most interesting valley of the Rhône, through which this river makes its way from the Vallais, or great valley above, among those mountains which seem to have shut up the Vallais, and through which the river must pass in running to the lake. M. de Saussure found some singular masses, which attracted his attention, in examining the structure of the rocks on the left side of this little valley. Like a true philosopher, and accurate naturalist, he desired to compare what was to be observed in the other side of this valley of the Rhône, which he had found so singular and so interesting on that which he had examined. Accordingly, in Spring 1785, he made a journey for that purpose. In this survey he found the most perfect correspondence between the two sides of this valley, so far as rocks of the same individual species, and precisely in the same order, are found upon the one side and upon the other.
This author, after describing those particular appearances, sums up the evidence which arises from this comparison of the two sides of the valley; and he here gives an example of just reasoning, of accuracy, and impartiality, which, independent of the subject, cannot be read without pleasure and approbation. But when it is considered, that here is a matter of the highest importance to the present theory, or to any other system of geology, no less than a demonstration that the rocks, of which the mountains on both sides of the valley of the Rhône are formed, are the same, and must have been originally continued in one mass, the following observations of our author will be most acceptable to every person who inclines to read upon this subject.
«§ 1079. On voit par cet exposé, que bien que la vallée du Rlione ait dans ce trajet près d'une lieue de largeur moyenne, les montagnes qui la bordent sont en general du même genre, et dans la même situation sur l'une et l'autre rive.
«Il y a cependant trois différences que je dois exposer et apprécier en peu de mots.
«La plus importante est dans ces couches de pierre calcaire, § 1073, que j'ai trouvées sur la rive droite, et que je n'ai point vue sur la gauche. Mais il est possible qu'elles y soient, et qu'elles m'ayent échappé, masquées par des débris ou par d'autre causes accidentelles; cette supposition est d'autant plus possible, que l'épaisseur de ces couches n'est que de quelques pieds. D'ailleurs il arrive souvent, que des filons, tel que paroît être celui dont je parle, ne s'étendent pas à de grandes distances, quoique la nature de la montagne demeure la même. Enfin ce qui diminue l'importance de cette différence, c'est que ces couches calcaires se trouvent dans le voisinage de l'ardoise qui passe, comme la pierre calcaire, pour une pierre de nature secondaire, et qui alterne très-fréquemment avec elle.
«Une autre difference que l'on aura pu remarquer, c'est que sur la rive droite, je n'ai point trouvé de petrosilex pur et en grandes masses, comme sur la rive gauche dans les environs de la cascade. Mais cette différence ne me frappe pas non plus beaucoup; parce qu'au lieu de petrosilex, j'ai trouvé sur la rive droite des roches composées en très-grande partie de feldspath. Or je regarde le pétrosilex et le feldspath comme des pierres de la même nature. Leur dureté est à-très-peu-près la même; leur densité la même, leur fusibilité la même; l'analyse chymique démontre dans l'un et dans l'autre les mêmes principes, la terre siliceuse, la terre argilleuse et le fer; et de plus ces ingrédiens s'y trouvent à très-peu-près dans les mêmes proportions. Il ne reste donc de différence que dans la couleur et dans l'agrégation des élémens. Or on fait que ces qualités accidentelles tiennent souvent à des causes qui peuvent être purement locales.
«La troisième différence, celle qui se trouve dans la direction de quelques-unes des couches, je l'ai déjà indiquée, § 1075. et il semble inutile de répéter, que quand des couches formées originairement dans une situation horizontale, ont été redressées par des opérations violentes de la nature, il n'y a pas lieu de s'étonner qu'elles n'aient pas exactement la même position dans tout l'espace qu'elles occupent.
«Les différences ne sont donc pas très-significantes, et les ressemblances sont au contraire du plus grand poids. Ce qui leur donne à mon gré la plus grande force, c'est la rareté des pierres dont ces montagnes sont composées, ces espèces de porphyres à base de pétrosilex, ces rochers feuilletées mélangées de feldspath et de mica; c'est encore la correspondance de l'ordre dans lequel elles se suivent; ces bancs de poudingues séparés par des ardoises sur une rive comme sur l'autre; leur situation également ou à-peu-près telle. Viola de grandes et fortes analogies et qui ne permettent pas de douter que ces montagnes, produites dans le même temps et par les mêmes causes, n'aient été anciennement unies.»
Having thus shown, that the Rhône had in the course of time hollowed out its way from the central mountain of the St. Gothard through the extensive valley of the Vallais we may still further trace the marks of its operation in the more open country towards the lake. It is an observation which M. de Saussure made in his way from the valley of the Rhône to Geneva.
«§ 1090. La grande route de Bex à Villeneuve suit toujours le fond de la vallée du Rhône, en côtoyant les montagnes qui bordent la droite ou le coté oriental de cette vallée. Ces montagnes sont en général de nature calcaire, mais on voit à leur pied, jusques auprès de la ville d'Aigle, située à une lieue et demi de Bex, la continuation des collines de gypse qui renferment les sources salées.
«§ 1091. A l'opposite de ces collines, au couchant de la grande route, on voit sortir du fond plat de la vallée deux collines allongées dans le sens de cette même vallée. Ces collines sont l'une et l'autre d'une pierre calcaire dure et escarpées presque de tous les côtes. L'une la plus voisine de Bex, ou la plus méridionale, se nomme Charpigny, l'autre Saint Tryphon.
«Il paroît évident que ces rochers isolés au milieu de cette large vallée sont de noyaux plus dures et plus solides qui ont résisté aux causes destructrices par lesquelles cette vallée à été creusé. Ils ne sont cependant pas exactement de la même nature, et surtout pas de la même structure; car celui de Saint Tryphon est composé de couches régulières, horizontales ou à-peu-près telles, tandis que celui de Charpigny a les siennes très-inclinées et souvent dans un grand désordre.»
In M. de Saussure's Journey to the Alps, we have now seen a description of the shape that had been given to things, by those operations in which strata had been consolidated and elevated above the sea; nothing but disorder and confusion seems to have presided in those causes, by which this mass of continent had been exposed to the sight of men; and nature, it would appear, had nothing in view besides the induration, the consolidation, and the elevation of that mass into the snowy regions of the atmosphere. From the descriptions now given, we see the operation of the waters upon the surface of the earth; we perceive a regular system of mountains and valleys, of rivulets and rivers, of fertile hills and plains, of all that is valuable to the life of man, and that which is still more valuable to man than life, viz. the knowledge of order in the works of nature, and the perception of beauty in the objects that surround him.
Let us now turn our view to distant regions, and see the effect of causes which, being general, must be every where perceived.
v2:26 «Discours sur l'Histoire Naturelle de la Suisse.»