Popular Science Monthly/Volume 14/February 1879/The Formation of Mountains
|THE FORMATION OF MOUNTAINS.|
PROFESSOR ALPHONSE FAVRE, of Geneva, has been making an interesting series of experiments to illustrate the formation of the great inequalities of the earth's surface by means of lateral thrust or crushing. These he describes and illustrates in a recent number of "La Nature," to which we are indebted for the illustrations which accompany this article. Professor Favre refers to the early experiments of Sir James Hall with various kinds of cloth, which he made to assume a variety of shapes by means of weights. He speaks of the various theories of the elevation of mountains, and especially of that of H. B. de Saussure, whose term refoulement seems to have meant much the same as that used by M. Favre, écrasement lateral.
"The three systems," M. Favre says, "which account for the origin of mountains by forces which push the great mineral masses from below upward, from above downward, or laterally, do not differ so much from each other as at first sight appears. Those geologists who have admitted the system of elevations as the principal cause of modification of the surface of the globe would probably enough admit the formation of depressions as a secondary modification; and so those who have accounted for these modifications mainly by depression, would probably enough also admit elevation as a secondary factor. Again, in the system of lateral crushing, there is a general depression of the surface of the earth, since there is a diminution in the length of the radius of our globe, and yet there result elevations of the ground in the midst of this general depression.
"The cause of lateral crushing," M. Favre goes on to say, "is owing to the cooling of the earth. It is, in fact, very probable that our globe is at the stage when, according to Élie de Beaumont, 'the mean annual cooling of the mass exceeds that of the surface, and exceeds it more and more.' It must follow that the external strata of the globe, tending always to rest on the internal parts, are wrinkled, folded, dislocated, depressed at certain points, and elevated at others."The experiments," M. Favre continues, "which I have made at the works of the Geneva Society for the manufacture of physical instruments, resemble much those of Sir James Hall; they differ notably, however, in two points: 1. The celebrated Scotchman caused the matter
which he wished to compress to rest on a body which itself could not be compressed, while I placed the layer of clay employed in these experiments on a sheet of caoutchouc, tightly stretched, to which I made it adhere as much as possible; then I allowed the caoutchouc to resume its original dimensions. By its contraction the caoutchouc would act equally on all points of the lower part of the clay, and more or less on all the mass in the direction of the lateral thrust. 2. Hall compressed, by a weight, the upper surface of the body which he wished to wrinkle, which prevented any deformation, while by leaving that surface free, I have seen, during the experiment, forms appear similar to those of hills and mountains which may be observed in various countries. . . .
"The arrangement of the apparatus is very simple. A sheet of India rubber 16 mm. in thickness, 12 cm. broad, and 40 cm. long, was stretched, in most of the experiments, to a length of 60 cm. This was covered with a layer of potter's clay in a pasty condition, the thickness of which varied, according to the experiments, from 25 to 60 mm. It will be seen from the dimensions indicated that pressure would diminish the length of the band of clay by one third. This pressure has been exerted on certain mountains of Savoy. For example, the section which I have given of the mountains situated between the Pointe-Percée and the neighborhood of Bonneville enables it to be seen that those folded and contorted strata which are shown between Dessy and the Col du Grand Bernard cover a length which is two thirds of that which they had before compression. These mountains, then, have been subjected, like the potter's clay, to a compression indicated by the ratio of 60 to 40. Contortions are not, perhaps, observed over all the surface of the globe; it has not been equally folded in all its extent, but they are found in a great number of countries, and even beneath strata almost horizontal. Sometimes the folds approach the vertical, and are close against each other; this structure indicates that pressure has been exercised in a stronger manner than I have indicated.
"These powerful lateral thrusts of the external and solid parts of the globe appear to result from a diminution which the radius of the interior pasty or fluid nucleus has undergone during millions of ages. It may have been sufficiently great to cause the solid crust (which must always have been supported on the interior nucleus, whose volume continually diminishes) to assume the forms which we know, with a slowness equal to that of the contraction of the radius.
"To return to my experiments. At the extremities of the band of clay are pieces of wood or supports, which accompany it in its movement of contraction. The clay is thus compressed at once by its adhesion to the caoutchouc and by lateral pressure of the supports. By the influence of the caoutchouc alone, without the presence of the supports, there are formed only slight wrinkles on the surface of a sheet of clay 3 or 4 cm. in thickness; and if the supports alone compressed the clay placed on a material which is not compressed (a very smooth oiled plate), the clay scarcely wrinkles near the center of its surface; it increases a little in thickness and forms swellings (bourrelets) against the supports. The strata which appear to divide the masses of clay, and which are represented in the figures, are not really strata, but simply horizontal lines at the surface of the clay."
Such pressure as has been applied in these experiments produces contortions of strata which elevate the surface of the matter compressed, as well in the plane parts or plains, as in those which take the forms of valleys, hills, or mountains. These latter have the appearance of vaults or folds, sometimes perpendicular, sometimes warped (déjetés); the ridges are complete, or broken at the summit by a longitudinal fracture, narrow below and wide above; next, another fracture, narrow above and wide below, is produced at the base of the mountain or vault. The sides of valleys are sometimes almost vertical, sometimes present gentle slopes. The strata are less strongly contorted in the lower parts than in the neighborhood of the upper surface. They are disjoined in certain parts by fissures or caverns; they are traversed by clefts or faults inclined or vertical. All these deformations
are the more varied in that they are not similar on the opposite sides of the same band of clay.
Most of these phenomena are seen in Fig. 1, which represents the result of an experiment made on a band of clay whose thickness before compression was about 25 mm., while after that it attained 62 mm. at the culminating point. At a is seen a vault a little broken at the summit, covering a cavern similar to that figured in the memoir of Sir J. Hall ("Transactions R. S. E.," vol. vii., 1813), and to that of the Petit Bernard in Savoy (Favre, "Recherches," pi. x.); at b is a valley open at one of its ends and almost closed at the other; at e is a vault almost straight, the prolongation of which is very level; at g, h, and l are vaults twisted and a little broken, while at i is a broken fold, the curves of which are almost vertical. All these accidents of the ground recall those which have been so often observed in the Jura, the Alps, and the Appalachians.
Fig. 2 represents a band of clay whose thickness was about 40 mm. before compression and 65 after. We remark contortions similar to those of the preceding figure, among others a vault a, very exactly formed. At distances are seen vertical slices, on which the pressure appears to have acted in a particularly energetic fashion, and which may be called "zones de refoulement"; the strata are there broken in an exceptional manner, often separated from each other. One of these vaults is replaced by a single vault on the opposite side of the band of clay.
Before compression, in the band of clay in Fig. 3, were seen the two divisions which are seen there now—that in the right was 33 cm. long and 25 mm. thick at a, and 35 at b; the left division was 25 cm. long and 65 mm. thick. A gentle slope united the part c to the part b. After compression, the mean height of a b was 45 and that of c 75 mm. All the layers were spread horizontally.
"In this experiment I have sought to imitate the effect of crushing at the limit of a mountain and a plain. The height of the mountain c has been notably increased, the five or six upper layers have advanced on the side of the plain; they encroach on it. The plain has, however, offered a resistance sufficiently great to cause the strata of the mountain to be strongly inflected at the bottom. From this struggle between the plain and the mountain there resulted a cushion, d, which is the first hill at the foot of the height. It also resulted that the strata of the plain assumed an appearance of depression at contact with the mountain in consequence of the vault which is formed at b; they plunge underneath the mountain. This resembles what is often seen in the Alps at the junction of the first calcareous chain and the hills of 'mollasse'; in fact, the strata of the latter rock seem to plunge under those of the neighboring heights. In consequence of the pressure, there are formed several ranges of hills in the plain between b and a.
"In Fig. 4 the band of clay had, before compression, a thickness of 45 mm.; after that the culminating point was more than 10 cm. I have here sought to represent what must happen when terrestrial pressure is exerted on horizontal strata still moist, deposited at the bottom of a sea where are two mountains already solidified. For this purpose I placed in the caoutchouc and under the clay two bare cylinders of wood, a and b, of about 35 mm. radius, at 20 cm. from the ends of the band of clay, and at the same distance from each other. Before compression the surface of the clay and the strata were completely horizontal. Pressure gave rise at the top of the half-cylinder, a, to a valley, c, formed by a twisting of the beds to the right, and by a little mountain, d, to the left. But I do not believe that it has ever been thought to assign to a valley an origin of this nature.
"On the other semi-cylinder, b, is produced an enormous elevation which has carried the ground to e, with such a rupture that the left lip, f, g, has suffered a complete reversal by turning, as on a hinge, around the horizontal line which passes by the point h. It follows that the four upper strata of clay designated by the figures 1, 2, 3, 4, being in a normal position before compression, are, after that, so arranged as to show the succession represented by the following arrangement of figures: 1, 2, 3, 4, 4, 3, 2, 1, 1, 2, 3, 4, making the section of this formation by a line drawn from x to z. If the left lip should disappear we should then have between the points x and z the section 1, 2, 3, 4, 5, 1, 2, 3, 4, 5. Sections analogous to these, presenting inversions in the order of strata, are known to geologists.
"The forms assumed by the clay depend on several circumstances which it is difficult to describe, such as the strength and the rate of compression, the thickness and the greater or less plasticity of the clay, etc. Why have accidents of the upper surface of the clay, which are intimately connected with those of the interior of the mass, so small an extension that they are not even similar in the two sides of a band of clay? This small continuity is owing to causes which we can neither foresee nor appreciate. Is it not the same in nature? Why is the chain of the Alps not a true chain, but a succession of masses often oblique with respect to each other? Why, in the Jura, do we see chains which have for their prolongation plains and valleys? It is always the case that the forms and structures obtained in these experiments have an incredible resemblance to those which are found on the surface of the globe. But it must be admitted that many of the latter have not been reproduced by these artificial crushings.
"It appears probable that, by pressures more powerful and more variedly employed, we might obtain again very different structures. But I have not thought it necessary to multiply these experiments, thinking that the varied forms which have resulted show sufficiently the effects of crushing."—Nature.
- "Bulletin Société Géologique de France," 1875, t. iii., pl. xxii. A. Favre, "Recherches Géologiques," Atlas, pl. ix.