« ForrigeFortsæt »
revolution itself must have occurred at a comparatively recent era. Moses, according to the chronological numbers of the Hebrew Pentateuch, places it 4171 years anterior to the present day*; or, according to what I deem the preferable chronological numbers of the Samaritan Pentateuch, 4761 years anterior to the same time : Mr. Cuvier, drawing his inference from the observation of actual phenomena, pronounces, that its epoch cannot bę dated much farther back than five or six thousand years.
The train of reasoning, through which he arrives at such a conclusion, is singularly curious and interesting,
By a careful investigation, says he, of what has taken place on the surface of the globe, since it has been laid dry for the last time, and since its continents have assumed their present form (at least in such parts as are somewhat elevated above the level of the ocean), it may be clearly seen, that this last revolution, and consequently the establishment of our exista ing societies, could not have been very ancient. This result is one of the best established, and least attended to, in rational zoology: and it is so much the more valuable, as it connects natural and civil history together in one uninterrupted series,
When we endeavour to estimate the quantity of effects, produced in a given time by any causes still acting, by comparing them with the effects which these
causes have produced since they began to operate, we may determine nearly the period at which their action commenced : which must necessarily be the same period, with that in which our continents assumed their present existing forms, or with that of the last retreat of the waters. It must have been since that last retreat of the waters, that the acclivities of our mountains have begun to disintegrate and to form slopes or taluses of the debris at their bottoms and upon their sides ; that our rivers have begun to flow in their present courses and to form alluvial depositions ; that our existing vegetation has begun to extend itself and to form vegetable soil; that our present cliffs or steep sloping coasts have begun to be worn away by the waters of the sea ; that our actual downs or sand-hills have begun to be blown away by the winds. and, dating from the same epoch, colonies of the human race must have then begun, for the first or for the second time, to spread themselves and to form new establishments in places fitted by nature for their reception.
De Luc and Dolomieu have most carefully examined the progress of the formation of new grounds by the collection of slime and sand washed down by the rivers: and, although exceedingly opposed to each other on many points of the theory of the earth, they agree exactly on this.
These formations augment very rapidly: they must have increased with the greatest rapidity at first, when the mountains furnished the greatest quantity of materials to the rivers : and yet their extent still continues to be extremely limited.
The memoir by Mr. Dolomieu, respecting Egypt,
tends to prove, that the tongue of land, on which Alerander caused his famous commercial city to be built, did not exist in the days of Homer: because they were then able to navigate directly from the island of Pharos into the gulf, afterwards called Lacus Mareotis ; and this gulf, as indicated by Menelaus, was between fifteen and twenty leagues in length. Supposing this to be accurate, it has only required the lapse of nine hundred years, from the days of Homer to the time of Strabo, to bring matters to the situation described by the latter author, when that gulf was reduced to the state of a lake only six leagues long.
It is a more certain fact, that, since that time, a still greater change has taken place. The sands, which have been thrown up by the sea and the winds, have formed, between the isle of Pharos and the site of ancient Alexandria, an isthmus more than four hundred yards broad, on which the modern city is now built. These collections of sand have also blocked ир the nearest mouth of the Nile, and have reduced the lake Mareotis almost to nothing ; while, in the course of the same period, the Nile has deposited alluvial formations all along the rest of the coast. In the time of Herodotus, the coast of the Delta extended in a straight line, and is even represented in that direction in the maps constructed for the geography of Ptolemy: but, since then, the coast has so far advanced as to have assumed a semicircular projection into the Mediterranean. We тау
learn in Holland and Italy, how rapidly the Rhine, the Po, and the Arno, since they have been confined within dikes, now elevate their beds, and push forward the alluvial grounds at their mouths toward the sea, forming long projecting promontories at their sides; and it may be concluded from this assured fact, that these rivers have not required the lapse of many centuries to deposit the low alluvial plains through which they now flow.
Many cities, which were flourishing sea-ports in well-known periods of history, are now several leagues inland; and some have even been ruined by this change. The inhabitants of Venice at present find it exceedingly difficult to preserve the lagunes, by which that once celebrated city is separated from the continent of Italy, from filling up: and there can be no doubt, that she will some day become united to the main land, in spite of every effort to preserve her insular situation.
We learn from Strabo, that Ravenna stood among lagunes in the time of Augustus, as Venice does now: but Ravenna is at present a league distant from the sea. Spina had been originally built by the Greeks on the sea-coast : but, in the time of Strabo, the sea was removed to the distance of ninety stadia. This city has been long since destroyed. Adria, which gave name to the Adriatic, was, somewhat more than twenty centuries ago, the chief port of that sea, from which it is now at the distance of six leagues. The. Abbè Fortis has even produced strong evidence for believing, that the Euganean hills may have been islands at a period somewhat more remote.
Mr. de Prony, having been directed by the French
government to examine and report upon the
precautions which might be employed for preventing the devastations occasioned by the floods of the Po, ascertained, that this river has so greatly raised the level of its bottom since it was shut in by dikes, that its present surface is higher than the roofs of the houses in Ferrara. At the same time, the alluvial additions produced by this river have advanced so rapidly into the sea, that, by comparing old charts with the present state, the coast appears to have gained no less than fourteen thousand yards since the year 1604, giving an average of an hundred and eighty to two hundred feet yearly. The Adige and the Po are both at présent higher than the intervening' lands: and the only remedy for preventing the disasters, which are now threatened by their annual overflowings, would be to open new channels for the more ready discharge of their waters through the low lands which have been formed by their alluvial depositions.
Similar causes have produced similar effects along the branches of the Rhine and the Maese; owing to which, all the richest districts of Holland have the frightful view of their great rivers held up by dikes, at the height of twenty or even thirty feet above the level of the land.
This formation and increase of new grounds, by alluvial depositions, proceeds with as much rapidity along the coasts of the North sea as on those of the Adriatic. These additions can be easily traced in Friesland and Groningen, where the epoch of the first dikes, constructed by the Spanish governor, Gaspard