Imágenes de páginas
PDF
EPUB

limestone occupies but a very limited space, and the first points, advancing to the east, where the carboniferous formation acquires any development, are on the great Bolivian plateau. M. d'Orbigny there observed several chains of it, such as the Apocheta de la Paz, the hills of Aja, and of Aygachi de las Penas, all the islands of Quebaya and of Periti in the lake of Titicaca, and more to the south the hills of Guallamarca and of Pucara, and some other patches. In general, the carboniferous strata are distributed chiefly to the east and to the west of the great Bolivian system, where they attain, especially to the east, an elevation of more than 13,000 feet. The carboniferous formation likewise forms, in the Chiquitian system, summits whose height is sometimes nearly 5000 feet, both in several chains of that system in the east and north of the province, and also more to the east in the province of Minas-Geraës.

The different beds united by M. d'Orbigny under the denomination of the carboniferous system are nevertheless divided, as I have already said, into two distinct series, the one consisting chiefly of limestones, and the other of sandstones; the first being the lower and fossiliferous, the last the upper and without fossils; and these two series, which occur united on the great Bolivian plateau, are elsewhere separated, for M. d'Orbigny found only the upper reddish sandstones, and never the limestones, to the east of the plateau, and on the Chiquitian system. There is therefore an important difference in position between these two series of beds. This difference might induce us to doubt whether the upper series is really to be regarded as belonging to the carboniferous system, and might lead us to refer it with as much probability to some one of the systems which succeed the carboniferous group in Europe, for example, to the red sandstone. The lower series is in fact the only one which can with great probability be referred to the carboniferous system. It is indeed only in the limestones, and in the calcareous sandstones of the inferior series, that M. d'Orbigny has found fossil organic bodies. He met with them at Yarbichambi, and in the islands of Quebaya and Periti, in the lake of Titicaca. The shells are in an excellent state of preservation, and retain all the necessary zoological characters.

These fossils belong to the genera Solarium or Euomphalus, Pleurotomaria, Natica, Pecten, Trigonia, Terebratula, Spirifer, Orthis, Leptaena, Productus, Turbinolia, Ceriopora, and Retopora. Of twenty-six species collected by M. d'Orbigny, twelve, or nearly the half, have the greatest analogy with the fossils of the carboniferous series of Europe, and of these, three, viz., Spirifer Pentlandi, Spirifer Roissyi, and Producíus Villiersi, are quite identical with the same species from Belgium and Russia. We have the same genera; also species having a common facies, and three of them quite identical. The whole facies of the fossils is so analogous, that at first sight we might imagine that we were looking at the usual species met with in the carboniferous rocks of Europe.

Among the fossils which do not belong to the carboniferous series of Europe we must remark a Trigonia (Trigonia antiqua), a genus which has not hitherto been noticed below the Jurassic formations. This curious discovery shews that M. d'Orbigny has understood how to ascertain, not only the resemblances of the American formations to ours, but also the differences between them,-differences which might well be expected at a distance 6000 miles, and which it is only surprising have not been found to be more considerable.

After the Silurian and Devonian periods, the American seas thus supported a different fauna from that of the two first epochs, and one completely analogous in character to that which lived during the carboniferous period in the seas of Europe. This analogy does not now exist between the faunas of the seas of Europe and of South America; and, as M. d'Orbigny remarks, it indicates, in the ancient geological periods, an uniformity of climate no longer observable. These inferences have so much the more weight, from being in this case supported on a triple basis. We have already spoken of the fossils which have induced M. d'Orbigny to refer the system of clay-slates of the Bolivian mountains to the Silurian system of Mr Murchison, and the system of quartzose sandstones to the Devonian system. Here, then, we have in South America, three members of the great palæozoic system, succeeding one another in the same order as the members of the same system in Europe, with which they have respectively the VOL. XXXVI. NO. LXXI.-JAN. 1844.

D

greatest analogy, Now, although we may retain some doubts. as to the exactitude of the identification of these different formations considered individually, it seems difficult to avoid regarding it as certain that the paleozoic system of South America corresponds as a whole with that of Europe, and that it is subdivided in an analogous manner. This great fact, which has been completely demonstrated by M. d'Orbigny, appears to us to be one of the most important with which geology has been enriched of late years.

Triassic System.-Succeeding the paleozoic formations, and immediately above the carboniferous sandstones of M. d'Orbigny, there is in South America a system of beds which he refers to the Trias of Europe, and which Mr Pentland* likewise considers as its representative. This identification we regard as indicated with plausibility by the observations made, but still not so rigorously established as that of the palæozoic system.

The presumed triassic deposits of Bolivia are composed of an alternation of magnesian limestones, of variegated clays, and of friable argillaceous sandstones. The lowest beds consist of a magnesian compact limestone, which is frequently divided into very thin waved laminæ. M. d'Orbigny has seen this member of the formation, having but a small thickness, near Laguillos, and in the valley of Miraflor. Above these limestones there lie, at the same localities, laminated rosecoloured or variegated clays, which often abound in crystals of gypsum. Above the clays, in the valley of Miraflor, there are compact magnesian limestones, in which M. d'Orbigny discovered numerous fossils, but unfortunately he is unable to particularize more than one species, the others having been lost. This species, the Chemnitzia potonensis, belongs to a new genus of turreted shells, which approaches the Melanias.

The rocks mentioned above are very analogous to those which constitute the trias group in Europe. The limestones of the Muschelkalk, in the north-east of France, and in the department of the Var, as well as the grès bigarrés of the

*It is much to be regretted that Mr Pentland has not yet published the valuable results of his extensive scientific investigations in South America.EDIT.

same districts, have recalled to M. d'Orbigny the aspect of the rocks of Bolivia, which he compares with them. The American formations, whose nature and position thus lead us to refer them, at least provisionally, to the Trias, but whose paleontological characters are still almost unknown, seem reduced to occupy, at the present day, and that in the form of large detached portions, the two slopes of the eastern Cordillera of the Bolivian system, and they there attain, at their highest elevation, a height of 4000 yards above the level of the sea. They are probably the remains of a great whole which covered the surface before the occurrence of the geological catastrophes that impressed the present forms on its external physiognomy.

Absence of the Jurassic System.-One of the most remarkable circumstances in American Geology is the absence of the Jurassic formations, a fact announced a long while ago by Von Buch. M. d'Orbigny did not find a single fossil which seems to belong to that period. The only exception to the general rule hitherto brought forward is, that M. d'Orbigny saw some Jurassic Terebratulæ among the fossils collected by M. Domeyko from a limestone in Chili.

Cretaceous Period.-The deposits of the cretaceous period seem, on the contrary, to have been very much developed on the American continent, as is proved by the collections of fossils made by Von Humboldt, Boussingault, Degenhardt, and by the geologists of Dumont d'Urville's last voyage, Doctors Hombron and Le Guillou. They occur from Columbia to Tierra del Fuego, or over the whole length of South America, whilst nevertheless they are interrupted in the middle. At that epoch there lived in America as in Europe, particular forms of Ammonite and Ancyloceras, &c.; and independently of the general resemblance of the forms, there were in Columbia and in the Parisian basin enough of identical species to induce us to suppose, that there was a direct communication between the European and the Columbian portions of the Cretaceous sea. It is well known that this sea formed in France two great distinct basins, the Parisian and the Mediterranean. It appears that this same sea covered with its waters not only a considerable portion of Columbia, but generally a large part of the regions situated on the north, the

west, and the south of the continent which then existed in those latitudes. The identity of the fossils of the chalk formation with those of the same formation in Europe, is not so great as regards the south of the American continent as it is in respect to the north, a circumstance which of course indicates a less direct communication. Perhaps we may infer the existence of some long piece of land, which continued as far as America the separation existing in Europe between the Parisian basin and the basin of the Mediterranean.

Tertiary System.-Another geological fact, and it is one of the most remarkable, is the immense extent of the Tertiary system in South America. When we compare it with the small basins disseminated throughout Europe, we are led to believe, along with M. d'Orbigny, that the smallness of the latter is an exception. The tertiary basin of the Pampas terminates and sinks under the Atlantic ocean, from the mouth of La Plata to the Straits of Magellan. Proceeding to the north from this last point, its limits, more or less remote from the Cordilleras, are still uncertain; but every thing leads us to believe, that the deposit of this epoch occupies the plains to the very base of the lateral chains. Advancing still farther to the north, the tertiary basin of the Pampas extends to the foot of the primitive hills of the province of Chiquitos; and it even appears that it is prolonged on all sides without interruption through these hills, into the great basin of the Amazon. Regarding only the portion situated to the south of the primitive hills of Chiquitos, the tertiary basin of the Pampas extends in the direction of the meridian, from the 17th to the 52d degree of south latitude, over a distance of upwards of 2400 miles. Its greatest breadth is about 800 miles.

Throughout this vast extent, and even at the foot of the northern declivity of the hills of Chiquitos, M. d'Orbigny has distinguished, in the American tertiary deposit, three different series, belonging to three successive epochs, viz. 1. The lower beds, destitute of organic remains, which he designates by the name of the Guaranian Tertiary Formation; 2. A middle portion, evidently marine, containing shells belonging to extinct species, and which he calls the Patagonian Tertiary Formation: and, 3. An upper portion, containing only skeletons

« AnteriorContinuar »