C.D. Warner, et al., comp.
The Library of the World’s Best Literature. An Anthology in Thirty Volumes. 1917.
Of Changes in the Structure of the Earth
By Georges Cuvier (1769–1832)
T
The time is past for ignorance to assert that these remains of organized bodies are mere lusus naturæ,—productions generated in the womb of the earth by its own creative powers. A nice and scrupulous comparison of their forms, of their contexture, and frequently even of their composition, cannot detect the slightest difference between these shells and the shells which still inhabit the sea. They have therefore once lived in the sea, and been deposited by it; the sea consequently must have rested in the places where the deposition has taken place. Hence it is evident the basin or reservoir containing the sea has undergone some change at least, either in extent, or in situation, or in both. Such is the result of the very first search, and of the most superficial examination.
The traces of revolutions become still more apparent and decisive when we ascend a little higher, and approach nearer to the foot of the great chains of mountains. There are still found many beds of shells; some of these are even larger and more solid; the shells are quite as numerous and as entirely preserved: but they are not of the same species with those which were found in the less elevated regions. The strata which contain them are not so generally horizontal; they have various degrees of inclination, and are sometimes situated vertically. While in the plains and low hills it was necessary to dig deep in order to detect the succession of the strata, here we perceive them by means of the valleys which time or violence has produced, and which disclose their edges to the eye of the observer. At the bottom of these declivities huge masses of their débris are collected, and form round hills, the height of which is augmented by the operation of every thaw and of every storm.
These inclined or vertical strata, which form the ridges of the secondary mountains, do not rest on the horizontal strata of the hills which are situated at their base and serve as their first steps; but on the contrary are situated underneath them. The latter are placed upon the declivities of the former. When we dig through the horizontal strata in the neighborhood of the inclined strata, the inclined strata are invariably found below. Nay sometimes, when the inclined strata are not too much elevated, their summit is surmounted by horizontal strata. The inclined strata are therefore more ancient than the horizontal strata. And as they must necessarily have been formed in a horizontal position, they have been subsequently shifted into their inclined or vertical position, and that too before the horizontal strata were placed above them.
Thus the sea, previous to the formation of the horizontal strata, had formed others which by some means have been broken, lifted up, and overturned in a thousand ways. There had therefore been also at least one change in the basin of that sea which preceded ours; it had also experienced at least one revolution: and as several of these inclined strata which it had formed first are elevated above the level of the horizontal strata which have succeeded and which surround them, this revolution, while it gave them their present inclination, had also caused them to project above the level of the sea so as to form islands, or at least rocks and inequalities; and this must have happened whether one of their edges was lifted up above the water, or the depression of the opposite edge caused the water to subside. This is the second result, not less obvious nor less clearly demonstrated than the first, to every one who will take the trouble of studying carefully the remains by which it is illustrated and proved.
If we institute a more detailed comparison between the various strata and those remains of animals which they contain, we shall soon discover still more numerous differences among them, indicating a proportional number of changes in their condition. The sea has not always deposited stony substances of the same kind. It has observed a regular succession as to the nature of its deposits: the more ancient the strata are, so much the more uniform and extensive are they; and the more recent they are, the more limited are they, and the more variation is observed in them at small distances. Thus the great catastrophes which have produced revolutions in the basin of the sea were preceded, accompanied, and followed by changes in the nature of the fluid and of the substances which it held in solution; and when the surface of the seas came to be divided by islands and projecting ridges, different changes took place in every separate basin.
Amidst these changes of the general fluid, it must have been almost impossible for the same kind of animals to continue to live; nor did they do so in fact. Their species, and even their genera, change with the strata: and though the same species occasionally recur at small distances, it is generally the case that the shells of the ancient strata have forms peculiar to themselves; that they gradually disappear, till they are not to be seen at all in the recent strata, still less in the existing seas, in which indeed we never discover their corresponding species, and where several, even of their genera, are not to be found; that on the contrary the shells of the recent strata resemble, as respects the genus, those which still exist in the sea; and that in the last formed and loosest of these strata there are some species which the eye of the most expert naturalists cannot distinguish from those which at present inhabit the ocean.
In animal nature, therefore, there has been a succession of changes corresponding to those which have taken place in the chemical nature of the fluid; and when the sea last receded from our continent, its inhabitants were not very different from those which it still continues to support.
Finally, if we examine with greater care these remains of organized bodies, we shall discover, in the midst even of the most ancient secondary strata, other strata that are crowded with animal or vegetable productions, which belong to the land and to fresh water; and amongst the most recent strata—that is, the strata which are nearest the surface—there are some of them in which land animals are buried under heaps of marine productions. Thus the various catastrophes of our planet have not only caused the different parts of our continent to rise by degrees from the basin of the sea, but it has also frequently happened that lands which had been laid dry have been again covered by the water, in consequence either of these lands sinking down below the level of the sea, or of the sea being raised above the level of the lands. The particular portions of the earth also, which the sea has abandoned by its last retreat, had been laid dry once before, and had at that time produced quadrupeds, birds, plants, and all kinds of terrestrial productions; it had then been inundated by the sea, which has since retired from it and left it to be occupied by its own proper inhabitants.
The changes which have taken place in the productions of the shelly strata, therefore, have not been entirely owing to a gradual and general retreat of the waters, but to successive irruptions and retreats, the final result of which, however, has been an universal depression of the level of the sea.
These repeated irruptions and retreats of the sea have been neither slow nor gradual; most of the catastrophes which have occasioned them have been sudden: and this is easily proved, especially with regard to the last of them, the traces of which are most conspicuous. In the northern regions it has left the carcasses of some large quadrupeds which the ice had arrested, and which are preserved even to the present day with their skin, their hair, and their flesh. If they had not been frozen as soon as killed, they must quickly have been decomposed by putrefaction. But this eternal frost could not have taken possession of the regions which these animals inhabited except by the same cause which destroyed them; this cause therefore must have been as sudden as its effect. The breaking to pieces and overturnings of the strata, which happened in former catastrophes, show plainly enough that they were sudden and violent like the last; and the heaps of débris and rounded pebbles which are found in various places among the solid strata demonstrate the vast force of the motions excited in the mass of waters by these overturnings. Life, therefore, has been often disturbed on this earth by terrible events: calamities which, at their commencement, have perhaps moved and overturned to a great depth the entire outer crust of the globe, but which, since these first commotions, have uniformly acted at a less depth and less generally. Numberless living beings have been the victims of these catastrophes; some have been destroyed by sudden inundations, others have been laid dry in consequence of the bottom of the seas being instantaneously elevated. Their races even have become extinct, and have left no memorial of them except some small fragment which the naturalist can scarcely recognize.
Such are the conclusions which necessarily result from the objects that we meet with at every step of our inquiry, and which we can always verify by examples drawn from almost every country. Every part of the globe bears the impress of these great and terrible events so distinctly, that they must be visible to all who are qualified to read their history in the remains which they have left behind.
But what is still more astonishing and not less certain, there have not been always living creatures on the earth, and it is easy for the observer to discover the period at which animal productions began to be deposited.
As we ascend to higher points of elevation, and advance towards the lofty summits of the mountains, the remains of marine animals—that multitude of shells we have spoken of—begin very soon to grow rare, and at length disappear altogether. We arrive at strata of a different nature, which contain no vestige at all of living creatures. Nevertheless their crystallization, and even the nature of their strata, show that they also have been formed in a fluid; their inclined position and their slopes show that they also have been moved and overturned; the oblique manner in which they sink under the shelly strata shows that they have been formed before these; and the height to which their bare and rugged tops are elevated above all the shelly strata, shows that their summits have never again been covered by the sea since they were raised up out of its bosom.
Such are those primitive or primordial mountains which traverse our continents in various directions, rising above the clouds, separating the basins of the rivers from one another, serving by means of their eternal snows as reservoirs for feeding the springs, and forming in some measure the skeleton, or as it were the rough framework of the earth. The sharp peaks and rugged indentations which mark their summits, and strike the eye at a great distance, are so many proofs of the violent manner in which they have been elevated. Their appearance in this respect is very different from that of the rounded mountains and the hills with flat surfaces, whose recently formed masses have always remained in the situation in which they were quietly deposited by the sea which last covered them.
These proofs become more obvious as we approach. The valleys have no longer those gently sloping sides, or those alternately salient and re-entrant angles opposite to one another, which seem to indicate the beds of ancient streams. They widen and contract without any general rule; their waters sometimes expand into lakes, and sometimes descend in torrents; and here and there the rocks, suddenly approaching from each side, form transverse dikes over which the waters fall in cataracts. The shattered strata of these valleys expose their edges on one side, and present on the other side large portions of their surface lying obliquely; they do not correspond in height, but those which on one side form the summit of the declivity often dip so deep on the other as to be altogether concealed.
Yet amidst all this confusion some naturalists have thought that they perceived a certain degree of order prevailing, and that among these immense beds of rocks, broken and overturned though they be, a regular succession is observed, which is nearly the same in all the different chains of mountains. According to them, the granite, which surmounts every other rock, also dips under every other rock; and is the most ancient of any that has yet been discovered in the place assigned it by nature. The central ridges of most of the mountain chains are composed of it; slaty rocks, such as clay slate, granular quartz (grès), and mica slate, rest upon its sides and form lateral chains; granular, foliated limestone or marble, and other calcareous rocks that do not contain shells, rest upon the slate, forming the exterior ranges, and are the last formations by which this ancient uninhabited sea seems to have prepared itself for the production of its beds of shells.
On all occasions, even in districts that lie at a distance from the great mountain chains, where the more recent strata have been digged through and the external covering of the earth penetrated to a considerable depth, nearly the same order of stratification has been found as that already described. The crystallized marbles never cover the shelly strata; the granite in mass never rests upon the crystallized marble, except in a few places where it seems to have been formed of granites of newer epochs. In one word, the foregoing arrangement appears to be general, and must therefore depend upon general causes, which have on all occasions exerted the same influence from one extremity of the earth to the other.
Hence it is impossible to deny that the waters of the sea have formerly, and for a long time, covered those masses of matter which now constitute our highest mountains; and farther, that these waters during a long time did not support any living bodies. Thus it has not been only since the commencement of animal life that these numerous changes and revolutions have taken place in the constitution of the external covering of our globe: for the masses formed previous to that event have suffered changes, as well as those which have been formed since; they have also suffered violent changes in their positions, and a part of these assuredly took place while they existed alone, and before they were covered over by the shelly masses. The proof of this lies in the overturnings, the disruptions, and the fissures which are observable in their strata, as well as in those of more recent formation, which are there even in greater number and better defined.
But these primitive masses have also suffered other revolutions, posterior to the formation of the secondary strata, and have perhaps given rise to, or at least have partaken of, some portion of the revolutions and changes which these latter strata have experienced. There are actually considerable portions of the primitive strata uncovered, although placed in lower situations than many of the secondary strata; and we cannot conceive how it should have so happened, unless the primitive strata in these places had forced themselves into view after the formation of those which are secondary. In some countries we find numerous and prodigiously large blocks of primitive substances scattered over the surface of the secondary strata, and separated by deep valleys from the peaks or ridges whence these blocks must have been derived. It is necessary, therefore, either that these blocks must have been thrown into those situations by means of eruptions, or that the valleys, which otherwise must have stopped their course, did not exist at the time of their being transported to their present sites.
Thus we have a collection of facts, a series of epochs anterior to the present time, and of which the successive steps may be ascertained with perfect certainty, although the periods which intervened cannot be determined with any degree of precision. These epochs form so many fixed points, answering as rules for directing our inquiries respecting this ancient chronology of the earth.