Theory of the Earth Volume Ii Part 15

Such mountains are necessarily composed of rounded ma.s.ses, 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 gra.s.s; 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 ma.s.s, or on the other hand a stratified ma.s.s, 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 ma.s.s 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 circ.u.mstances of the place would permit.

Therefore, whether we suppose the mountains formed of a rock in ma.s.s, 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 a.s.sociated, 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 a.s.sociated 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 a.n.a.logy, of the mountains formed of the same materials, and thus a.s.sociated. The highest of those mountains should be near the center of the ma.s.s; but, in extensive ma.s.ses of this kind, there may also be more than one center. Nor are all the a.s.sociated 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 ma.s.ses 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 ma.s.s 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 ma.s.ses 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 ma.s.s 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 ma.s.s; the other is the degradation of that ma.s.s, 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 ma.s.s 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 ma.s.s; but on the south and north sides of this central ma.s.s, the basaltic matters had been forced also in those inclined beds among the regular strata. On the north side we find remarkable ma.s.ses 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 pa.s.s. 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 Rhone, 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 Rhone divides in issuing from the Alps into the plain, had been also united, in forming one continued ma.s.s 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 Rhone 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 separent de la grande route, sont composees d'un melange tres ressemblant au precedent, et ce sont-la, les derniers rochers primitifs que l'on rencontre en sortant des Alpes par cette vallee. Le village de Juviana, dont ils occupent les derrieres, est encore a une lieue de St Maurice.

-- 1062. A l'extremite de ces rochers, on voit une grande ravine, ou plutot une vallee ouverte du nord au midi, dans laquelle coule le torrent de St. Barthelemi. Cette vallee termine les montagnes primitives que je viens de decrire: au-dela commencent les montagnes calcaires.

Cependant le pied de la montagne primitive, coupe par le torrent, est demeure engage sous les premieres couches de la montagne calcaire.

Au travers de cette vallee, on voit de hautes montagnes couverte de neige, situees 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 cote du Rhone, on voit une autre cime aussi tres-elevee, 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 meme de matiere tout-a-fait singuliere. L'une et l'autre presentent leurs escarpemens a la vallee du Rhone. Leurs cimes crenelees sont de la meme couleur brune. Sous ces cimes brunes, on voit de part et d'autre une bande grise, qui paroit horizontale, et au-dessous de cette bande grise, le rocher, dans l'une comme dans l'autre, reprend sa couleur jaunatre. Ces montagnes sont surement secondaires, les bandes grises paroissent etre de pierre a chaux, et les jaunes de schiste argilleux et de gres, a en juger du moins a cette distance, car je ne les ai point observees de plus pres. Elles paroissent aussi appartenir a des chaines secondaires qui pa.s.sent derriere les chaines primitives, que nous avons observees sur les bords du Rhone, et quoique les bandes jaunes et grises que l'on y observe, semblent horizontales, je ne doute point que les couches memes, dont ces bandes sont les sections, ne descendent en arriere avec a.s.sez de rapidite; le escarpemens de ces montagnes en font une preuve a-peu-pres certaine.

Ces hautes montagnes auroient-elles ete anciennement liees entr'elles par des intermediaires de la meme nature, que couvroient, et les primitives que nous avons observees, et toute cette vallee dans laquelle coule aujourd'hui le Rhone? Je me garderois bien de l'affirmer, mais je ferois tente de le croire.

-- 1063. Depuis la vallee dont je viens de parler, et qui termine au couchant les montagnes primitives, celles qui suivent jusques a St.

Maurice, sont de nature calcaire a couches epaisses et suivies. Ces couches s'elevent contre les primitives que nous avons cotoyees; et celles qui en sont les plus voisines paroissent fort tourmentees; ici flechies, la rompues. Apres une interruption, ces rochers sont suivis d'autres rochers, aussi calcaires, coupes a pic du cote de la vallee, et composes de grandes a.s.sises horizontales. Ces rochers forment une enceinte demi-circulaire, qui vient presque se joindre a ceux qui bordent la rive droite du Rhone, et former ainsi l'entree de cette vallee, dont le fleuve ne sort que par une issue tres-etroite.

La ville de St. Maurice est ainsi renfermee par cette enceinte de rochers, dont les bancs epais, bien suivis, separes par des cordons de verdure, et couronnes par des forets, avec un hermitage niche entre ces bancs, presente une aspect singulier et pittoresque.

-- 1064. Les rochers correspondans de l'autre cote du Rhone, 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 composee de couches contournees, froissees et repliees de la maniere la plus etrange. Ce qu'il y a encore de remarquable, c'est que ces couches ainsi repliees en ont d'autres a cote d'elles qui sont planes, presque verticales, et d'autres sous elles, qui sont horizontales. Il faudroit avoir observe de pres ce singulier rocher, et avoir determine comment et jusqu'a quel point ces couches sont unies entr'elles pour former les conjectures sur leur origine. Car la vallee est trop large pour que l'on puisse en juger avec precision d'une rive a l'autre.

On voit avec peine que cette large vallee soit aussi peu cultivee; elle est presque partout couverte, ou de marais, ou de debris des montagnes voisines.

-- 1065. Avant de quitter cette vallee, je jetterai un coup-d'oeil general sur la singuliere suite de rochers qui composent la chaine que nous venons d'observer.

Les deux extremites sont calcaires, avec cette difference, que celle qui est la plus pres de Martigny est melee de mica, tandis que celle de St Maurice n'en contient point. Entre ces calcaires sont refermees 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 cla.s.se parmi les montagnes tertiaires, ou de la formation la plus recente. Mais ces poudingues-ci, qui ne contiennent aucun fragment de pierre calcaire, qui ne sont meme point unis par un gluten calcaire, ne sont vraisemblablement pas posterieures a la formation des montagnes calcaires, ou du moins ils ne doivent point etre confondus avec ces gres et ces poudingues de formation nouvelle, qui entrent dans la composition des montagnes du troisieme ordre.

Quant aux ardoises que se trouvent interposees au milieu de ces gres et de ces poudingues, -- 1054, elles sont de nature argilleuse, et dans l'ordre des pierres que l'on nomme secondaires.

Ces ardoises, de meme 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 ete 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 Rhone, 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 pa.s.s in running to the lake. M. de Saussure found some singular ma.s.ses, 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 Rhone, 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 Rhone are formed, are the same, and must have been originally continued in one ma.s.s, 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 expose, que bien que la vallee du Rlione ait dans ce trajet pres d'une lieue de largeur moyenne, les montagnes qui la bordent sont en general du meme genre, et dans la meme situation sur l'une et l'autre rive.

Il y a cependant trois differences que je dois exposer et apprecier en peu de mots.

La plus importante est dans ces couches de pierre calcaire, -- 1073, que j'ai trouvees 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 echappe, masquees par des debris ou par d'autre causes accidentelles; cette supposition est d'autant plus possible, que l'epaisseur de ces couches n'est que de quelques pieds. D'ailleurs il arrive souvent, que des filons, tel que paroit etre celui dont je parle, ne s'etendent pas a de grandes distances, quoique la nature de la montagne demeure la meme.

Enfin ce qui diminue l'importance de cette difference, c'est que ces couches calcaires se trouvent dans le voisinage de l'ardoise qui pa.s.se, comme la pierre calcaire, pour une pierre de nature secondaire, et qui alterne tres-frequemment avec elle.

Une autre difference que l'on aura pu remarquer, c'est que sur la rive droite, je n'ai point trouve de petrosilex pur et en grandes ma.s.ses, comme sur la rive gauche dans les environs de la cascade. Mais cette difference ne me frappe pas non plus beaucoup; parce qu'au lieu de petrosilex, j'ai trouve sur la rive droite des roches composees en tres-grande partie de feldspath. Or je regarde le petrosilex et le feldspath comme des pierres de la meme nature. Leur durete est a-tres-peu-pres la meme; leur densite la meme, leur fusibilite la meme; l'a.n.a.lyse chymique demontre dans l'un et dans l'autre les memes principes, la terre siliceuse, la terre argilleuse et le fer; et de plus ces ingrediens s'y trouvent a tres-peu-pres dans les memes proportions.

Il ne reste donc de difference que dans la couleur et dans l'agregation des elemens. Or on fait que ces qualites accidentelles tiennent souvent a des causes qui peuvent etre purement locales.

La troisieme difference, celle qui se trouve dans la direction de quelques-unes des couches, je l'ai deja indiquee, -- 1075. et il semble inutile de repeter, que quand des couches formees originairement dans une situation horizontale, ont ete redressees par des operations violentes de la nature, il n'y a pas lieu de s'etonner qu'elles n'aient pas exactement la meme position dans tout l'es.p.a.ce qu'elles occupent.

Les differences ne sont donc pas tres-significantes, et les ressemblances sont au contraire du plus grand poids. Ce qui leur donne a mon gre la plus grande force, c'est la rarete des pierres dont ces montagnes sont composees, ces especes de porphyres a base de petrosilex, ces rochers feuilletees melangees de feldspath et de mica; c'est encore la correspondance de l'ordre dans lequel elles se suivent; ces bancs de poudingues separes par des ardoises sur une rive comme sur l'autre; leur situation egalement ou a-peu-pres telle. Viola de grandes et fortes a.n.a.logies et qui ne permettent pas de douter que ces montagnes, produites dans le meme temps et par les memes causes, n'aient ete anciennement unies.

Having thus shown, that the Rhone 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 Rhone to Geneva.

-- 1090. La grande route de Bex a Villeneuve suit toujours le fond de la vallee du Rhone, en cotoyant les montagnes qui bordent la droite ou le cote oriental de cette vallee. Ces montagnes sont en general de nature calcaire, mais on voit a leur pied, jusques aupres de la ville d'Aigle, situee a une lieue et demi de Bex, la continuation des collines de gypse qui renferment les sources salees.

-- 1091. A l'opposite de ces collines, au couchant de la grande route, on voit sortir du fond plat de la vallee deux collines allongees dans le sens de cette meme vallee. Ces collines sont l'une et l'autre d'une pierre calcaire dure et escarpees presque de tous les cotes. L'une la plus voisine de Bex, ou la plus meridionale, se nomme _Charpigny_, l'autre _Saint Tryphon._

Il paroit evident que ces rochers isoles au milieu de cette large vallee sont de noyaux plus dures et plus solides qui ont resiste aux causes destructrices par lesquelles cette vallee a ete creuse. Ils ne sont cependant pas exactement de la meme nature, et surtout pas de la meme structure; car celui de _Saint Tryphon_ est compose de couches regulieres, horizontales ou a-peu-pres telles, tandis que celui de _Charpigny_ a les siennes tres-inclinees et souvent dans un grand desordre.

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 ma.s.s 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 ma.s.s 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.

CHAP. XII.

_The Theory ill.u.s.trated, by adducing examples from the different Quarters of the Globe_