Imágenes de página
PDF
ePub

have not studied the subject, geologists entertain no doubt that all our present mountains, composed of sedimentary matter, were accumulated beneath the sea during countless ages; and, if so, other continents must have existed to furnish materials, though no traces of such lands now remain."— Sir R. I. Murchison's Silurian System, p. 573.

"It is universally acknowledged among geologists that these immense sedimentary deposits could only have accumulated beneath the waters of the ocean during an incalculable period of time, long anterior to the present condition of the surface. Now, in order to furnish materials for such formations we must conceive of the existence of continents where no vestige of them now remains; from the abrasion and destruction of these, and from the transporting power of river and ocean currents, the materials composing them were reduced to the state of pebbles, sand, and finely comminuted mud, which were widely diffused, and gradually or rapidly precipitated upon the ocean bed."—Hall's Geology of Western New York, p. 521.

If no trace of those continents now remains, it is plainly impossible, from the mere strata themselves, to demonstrate that they were in such positions and consisted of such elements, that they can have furnished the materials from which the strata were formed. It is only by assuming the point to be proved, that all sedimentary strata must have been formed of materials derived from pre-existing mountains and dry land, that such a conclusion can be obtained. Discard that assumption, and let the question to be determined be, whether the materials of stratified rocks must necessarily be derived by disintegration and transportation from granitic continents and mountains, and the error of their argument is apparent; as it is on the assumption that that must be their origin that their whole theory is founded.

Some geologists seem to suppose that a large share at least of those materials were derived from the mountains that now subsist on the globe. But it is shown to be erroneous by the fact that all the great ranges, and most of every subordinate class, have been thrown up from beneath the ocean since the formation of the tertiary, the last great division of the strata.*

"If we admit that the primary, the transition, the secondary, and the tertiary classes of rock were formed at

*"If we date the age of granite from the period of the elevation of granite mountains we must admit that some granite mountains are comparatively recent, for they have been elevated since the deposition of the secondary strata. I have shown this to be the case with the Bernese and Savoy Alps in my Travels published in 1827. In the edition of the present work in 1828, 1 have shown also, by a description and sections, that the elevation of the granite of Savoy is more recent than that of the central part of England. M. Elie de Beaumont has since adopted the same views, and has extended them to other mountain ranges. Professor Sedgwick and Mr. Murchison have further proved that a part of the Tyrolean and Bavarian Alps was elevated since the deposition of tertiary strata; for these strata are filled up with them to the height of several thousand feet."— Bakewell's Geology, p. 101.

different successive epochs, and that the lower beds in each of these classes are more ancient than the beds that rest upon them, it follows as a necessary consequence that the elevation of any of these rocks must be dated from a later epoch than the period of their formation. The elevation of a range of primary or transition mountains, if they are not covered by any secondary or tertiary formations, may indeed be dated either from an epoch coeval with their consolidation, or from any subsequent epoch; but if they are partly covered by secondary or tertiary beds which are tilted up with them, we have direct evidence that the date of their elevation was posterior to the secondary or tertiary epoch."—BakeuoelUs Geology, p. 101.

"It is a general law, confirmed by most ample evidence, that the interior parts of mountainous regions consist of granite and other pyrogenous rocks rising from below all the strata, and bearing them up to their present elevations. From these elevated points and lines, both the subjacent igneous and the superior stratified rocks descend at various angles towards the plains and more level regions, beneath which they sink and pass at various distances until they again emerge in some other mountain group having similar characters. In consequence of this arrangement, it happens generally that the oldest strata, those which sink deepest under the plains, rise highest against the mountain slopes.

. . . . The most constant of all the facts connected with this part of the subject, is the development of granitic or some other pyrogenous rocks about the centres of the elevated groups from beneath all the strata there occurring."—Phillips's Guide, p. 31.

"Etna would appear to have been the seat of volcanic action through a long series of ages, commencing with the supercretaceous rocks, on which much of the igneous mass is now based.

"In central France, amid the extinct volcanoes which there constitute such a remarkable feature in the physical geography of the country, we certainly approach relative dates in some instances. Thus the volcanic mass of the Plomb du Cantal appears to have burst through, to have upset, and to have fractured the fresh water limestones of the Cantal, which, according to Messrs. Lyell and Murchison, may be equivalent to the fresh-water deposits of the Paris basin, and to those of Hampshire and the Isle of Wight.

"With regard to the igneous rocks of Auvergne, MM. Croiset and Jobert consider that there are about thirty beds above the fresh-water limestone, which may be divided into four alternations of alluvial detritus and basaltic deposits. Among the beds there are three which contain organic remains; two belong to the third of the ancient alluvions, that which succeeded the second epoch of volcanic eruptions ; the third fossiliferous deposit being referable to the last epoch of ancient alluvion.

"The principal ossiferous bed is about nine or ten feet thick, and can be traced a considerable distance. . . . The fossil species, according to MM. C. and J., are very numerous, consisting of Elephant, Mastodon, Hippopotamus, Rhinoceros, Tapir, Boar, Felis, Hyaena, Bear, Canis, Castor, Hare, Water Rat, Deer, and Ox."—H. T. Be la Beche's Manual, pp. 241, 242.

"The same phenomena are exhibited in the Alps on a much grander scale; those mountains being composed, in some even of their higher regions, of newer secondary formations, while they are encircled by a great zone of tertiary rocks of different ages, both on the southern flanks towards the plains of the Po, and on the side of Switzerland and Austria, and at their eastern termination towards Styria and Hungary. This tertiary zone marks the position of former seas or gulfs, like the Adriatic, which were many thousand feet deep, and wherein masses of strata accumulated, some single groups of which seem scarcely inferior in thickness to the whole of our secondary formations in England. These marine tertiary strata have been raised to the height of from 2,000 to 4,000 feet, and consist of formations of different ages, characterized by different assemblages of organized fossils. The older tertiary groups generally rise to the greatest heights, and form interior zones nearest to the central ridges of the Alps. Although we have not yet ascertained the number of different periods at which the Alps gained accessions to their height and width, yet we can affirm that the last series of movements occurred when the seas were inhabited by many existing species of animals.

"The Pyrenees also have acquired the whole of their present altitude, which in Mount Perdu exceeds 11,000 feet, since the deposition of some of the newer or cretaceous members of our secondary series."—Lyell's Principles, vol. i., p. 139.

There are similar proofs, also, of the elevation from

« AnteriorContinuar »