Читать книгу Essay on the Theory of the Earth - Georges baron Cuvier - Страница 41

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MM. Deluc and Dolomieu have most carefully examined the progress of the formation of new ground by means of matters washed down by rivers; and although exceedingly opposed to each other on many points of the Theory of the Earth, they agree in this. These formations augment very rapidly; they must have increased still more rapidly at first, when the mountains furnished more materials to the rivers, and yet their extent is still inconsiderable.

Dolomieu’s Memoir respecting Egypt[90] tends to prove, that the tongue of land on which Alexander caused his city to be built, did not exist in the days of Homer; that they were then able to navigate directly from the island of Pharos into the gulf afterwards called Lake Mareotis; and that this gulf was then, as indicated by Menelaus, from fifteen to twenty leagues in length. It had, therefore, only required the nine hundred years that elapsed between the time of Homer and that of Strabo, to bring things to the state in which this latter author describes them, and to reduce the gulf in question to the form of a lake, of six leagues in length. It is more certain, that, since that time, things have changed still more. The sand thrown up by the sea and winds have formed, between the island of Pharos and the site of ancient Alexandria, a tongue of land two hundred fathoms in breadth, upon which the modern city has been built. It has blocked up the nearest mouth of the Nile, and reduced the lake Mareotis to almost nothing; while, during the same period, the alluvial matter carried down by the Nile, has been deposited along the rest of the shore, and has greatly increased its extent.

The ancients were not ignorant of these changes. Herodotus says, that the Egyptian priests regarded their country as a gift of the Nile. It is only in a manner, he adds, within a short period, that the Delta has appeared[91]. Aristotle observes, that Homer speaks of Thebes as if it had been the only great city in Egypt; and nowhere makes mention of Memphis[92]. The Canopian and Pelusian mouths of the Nile were formerly the principal ones; and the coast extended in a straight line from the one to the other; and in this manner it still appears in the charts of Ptolemy. Since then, the water has been directed into the Bolbitian and Phatnitic mouths; and it is at these entrances into the sea that the greatest depositions have been formed, which have given the coast a semicircular outline. The cities of Rosetta and Damieta, which were built upon these mouths, close to the edge of the sea, less than a thousand years ago, are now two leagues distant from it. According to Demaillet[93], it would only have required twenty-six years to form a promontory of half a league in extent before Rosetta.

An elevation is produced in the soil of Egypt, at the same time that this extension of its surface takes place, and the bed of the river rises in the same proportion as the adjacent plains, which makes the inundations of every succeeding century pass far beyond the marks which it had left during the preceding ones. According to Herodotus, a period of nine hundred years was sufficient to establish a difference of level amounting to ten or twelve feet. At Elephantia[94], the inundation at present exceeds by seven feet the greatest heights which it attained under Septimus Severus, at the commencement of the third century. At Cairo, before it is judged sufficient for the purpose of irrigation, it must exceed, by three feet and a half, the height which was necessary in the ninth century. The ancient monuments of this celebrated land have all their bases more or less buried in the soil. The mud left by the river even covers, to a depth of several feet, the artificial mounds on which the ancient towns were built[95].

The delta of the Rhone is not less remarkable for its increase. Astruc gives a detailed account of it in his Natural History of Languedoc; and proves, by a careful comparison of the descriptions of Mela, Strabo and Pliny, with the state of the places as they existed at the commencement of the eighteenth century, taking into account the statements of several writers of the middle age, that the arms of the Rhone have increased three leagues in length in the course of eighteen hundred years; that similar additions of land are made to the west of the Rhone; and that a number of places, which were situated, six or eight hundred years ago, at the edge of the sea or of large pools, are now several miles distant from the water.

Any one may observe in Holland and Italy, with what rapidity the Rhine, the Po, and the Arno, since they have been confined within dikes, raise their beds, advance their mouths into the sea, forming long promontories at their sides; and judge, from these facts, how small a number of ages was required by these rivers to deposit the low plains which they now traverse.

Many cities, which were flourishing sea-ports at well known periods of history, are now some leagues inland; and several have even been ruined, in consequence of this change of position. The inhabitants of Venice find it exceedingly difficult to preserve the lagunes, by which that city is separated from the continent; and notwithstanding all their efforts, it will be inevitably joined to the mainland[96].

We know, from the testimony of Strabo, that Ravenna stood among lagunes in the time of Augustus, as Venice does now; but at present Ravenna is a league distant from the shore. Spina had been built by the Greeks at the edge of the sea; yet in Strabo’s time it was ninety stadia from it, and is now destroyed. Adria in Lombardy, which gave name to the Adriatic sea, and of which it was, somewhat more than twenty centuries ago, the principal port, is now six leagues distant from it. Fortis has even rendered it probable that, at a more remote period, the Euganian Mountains may have been islands.

M. de Prony, a learned member of the Institute, and inspector-general of bridges and roads, has communicated to me some observations which are of the greatest importance, as explaining those changes that have taken place along the shores of the Adriatic[97]. Having been directed by government to investigate the remedies that might be applied to the devastations occasioned by the floods of the Po, he ascertained that this river, since the period when it was shut in by dikes, has so greatly raised the level of its bottom, that the surface of its waters is now higher than the roofs of the houses in Ferrara. At the same time, its alluvial depositions have advanced so rapidly into the sea, that, by comparing old charts with the present state, the shore is found to have gained more than six thousand fathoms since 1604, giving an average of a hundred and fifty or a hundred and eighty, and in some places two hundred feet yearly. The Adige and the Po, are at the present day higher than the whole tract of land that lies between them; and it is only by opening new channels for them in the low grounds, which they have formerly deposited, that the disasters which they now threaten may be averted.

The same causes have produced the same effects along the branches of the Rhine and the Meuse; and thus the richest districts of Holland have continually the frightful view of their rivers held up by embankments at a height of from twenty to thirty feet above the level of the land.

M. Wiebeking, director of bridges and highways in the kingdom of Bavaria, has written a memoir upon this subject, so important as to be worthy of being properly understood, both by the people and the government, in all countries where these changes take place. In this memoir, he shews that the property of raising the level of their beds is common in a greater or less degree to all rivers.

The additions of land that have been made along the shores of the North Sea, have not been less rapid in their progress than in Italy. They can be easily traced in Friesland and in the country of Groningen, where the epoch of the first dikes, constructed by the Spanish governor Gaspar Roblès, is well known to have been in 1570. An hundred years afterwards, land had already been gained, in some places, to the extent of three quarters of a league beyond these dikes; and even the city of Groningen, partly built upon the old land, on a limestone which does not belong to the present sea, and in which the same shells are found as in the coarse limestone of the neighbourhood of Paris, is only six leagues from the sea. Having been upon the spot, I am enabled to adduce my own testimony in confirmation of facts already well known, and which have been so well stated by M. Deluc[98]. The same phenomenon may be as distinctly observed along the coasts of East Friesland, and the countries of Bremen and Holstein, as the period at which the new grounds were inclosed for the first time is known, and the extent that has been gained since can be measured. This new alluvial land, formed by the rivers and the sea, is of astonishing fertility, and is so much the more valuable, as the ancient soil of these countries, being covered with heaths and peat-mosses, is almost everywhere unfit for cultivation. The alluvial lands alone produce subsistence for the many populous cities that have been built along these coasts, since the middle age, and which perhaps would not have attained their present flourishing condition, without the aid of the rich deposits which the rivers had prepared for them, and which they are continually augmenting.

If the size which Herodotus attributes to the Sea of Asoph, which he makes equal to the Euxine[99], had been less vaguely indicated, and if we knew precisely what he meant by the Gerrhus[100], we should there find strong additional proofs of the changes produced by rivers, and the rapidity with which they are made; for the alluvial depositions of rivers alone have, since the time of Herodotus, that is to say, in the course of two thousand and two or three hundred years, reduced the Sea of Asoph[101] to its present comparatively small size, shut up the course of the Gerrhus, or that branch of the Dnieper which had formerly joined the Hypacyris, and discharged its waters along with that river into the gulf called Carcinites, now the Olu-Degnitz, and reduced the Hypacyris itself to almost nothing[102]. We should possess proofs no less strong of the same kind, could we be certain that the Oxus or Sihoun, which at present discharges itself into the lake Aral, formerly reached the Caspian Sea. But we are in possession of facts sufficiently conclusive on the point in question, without adducing such as are doubtful, and without being exposed to the necessity of making the ignorance of the ancients in geography the basis of our physical propositions.[103]