XVIII. BOOK sides of mountains, and frequently containing in the centre a rounded stone. The traveller Kalm observed these hollows in the United States, near Fort Nicholson. Several examples, observed in Switzerland and in Siberia, might be mentioned, but they are to be met with only in sulphated calcareous rock:* on the contrary, those of Sweden and the United States exist in granite. According to Bergmann, they have been formed by running waters, which had become engulphed there, and had imparted a rotatory motion to a stone detached from a neighbouring rock.† Drying up of the lakes. In admitting this explanation, the giants' cauldrons would incontestibly prove, that there was a time when the granite was not harder than common limestone or gypsum. There are many lakes which hold in solution saline, earthy, metallic, and bituminous substances: these substances form sometimes simple deposites; at other times, they are thrown down to the bottom of the lake, by chemical precipitation. Several causes may here concur. The different gases and acids with which the waters of these lakes are charged, may give rise to crystallizations; a cooling in lakes which possess a certain degree of heat, may also have the same effect. It is certain, that the remains of the beings that live and die in these waters, form beds of calcareous earth. Trees and vegetables, which are carried along by the waters, form floating islands, which, by degrees, unite together, and either cover the lakes with a crust of earth, or, by sinking, raise up the bottom. We must add, that the more a stagnant mass of water loses in depth, the more it evaporates; for the evaporation of water is always in the direct ratio of its surface, and in the inverse ratio of its depth, as experiments have proved. There are in all mountainous and marshy countries numerous instances of small lakes which have been dried up by one of these causes; we may therefore safely infer, that the same phenomena Patrin, Histoire Naturelle des Minéraux, iii. 201. Saussure, Voyages, sect. 1238. + Bergmann, Géog. Phys. ii. 228. XVIII. have taken place upon a larger scale, and that the formation BOOK of several extensive plains is to be ascribed to the drying up of inland seas, or great lakes; as for example, part of those which border on the Caspian Sea.* down of dams. The damming up of a lake commences by confining its Breaking basin, but the lake always receiving the same quantity of water, it necessarily overflows, and if it happens to discharge itself into other inferior lakes, it may produce successive overflowings, which change the surface of the surrounding country. These efforts would be more violent if a sudden thaw, and a superabundance of water, took place at the same time. The waters in this case would perhaps be impelled by so impetuous a force, as to overthrow every thing they met with in their course, and make large excavations in the chains of mountains, and thus form great valleys. This idea was developed by Sulzer, and carried to extremes by Lamanon. It occurs very naturally, when one considers the great lakes of North America, which flow into one another, and finally into the river Saint Lawrence. If the soil which borders the Ohio, and where we find the bones of the mammoth, is impregnated with salt; if the plain which surrounds the river of the Amazons, is, as is said, entirely composed of soft earth, without any stones; if we find in the plain of Crau and in Dauphiny the same pebbles and rounded flints, as upon the borders of the Lake of Geneva, then all these circumstances may be accounted for on the supposition of the overflowing of the interior lakes ;— but historical certainty is wanting, and will always be wanting, to these hypotheses. It is, besides, very certain, that valleys must have existed before the flowing of the waters could take place. All, therefore, which can be conceded to this theory of the excavation of the earth by currents of water, must be limited to effects which regard only the superficial strata. * Delamétherie, Théorie de la Terre, sect. 1417. Lamanon, Journal de Physique, 1780, December, p. 474, BOOK We come to a question which has greatly engaged the XVIII. attention of physical geographers: the diminution of the waters of the sea. Do the waters of the nish? Let us begin by defining the subject of inquiry. We have sea dimi nothing to do with those marine or other fluids, which at different epochs covered the globe, and in which lived innumerable shell fishes, sea petrifactions, and polypuses, which we have seen upon the summits of the highest mountains. It is plain that these fluids have disappeared; but the slow or rapid manner of this disappearance, and its internal or external causes, can be known to us only by conjecture. Man existed not at the time of those revolutions which have heaped up on one another the remains of the sea, and of fresh water animals. The question is only concerning changes which the sea of the terrestrial globe may have undergone since the last revolution, which has created our existing continents. To the question thus limited an answer may be given. A very long experience, that of more than twenty centuries, enlightened by the torch of history, seems to prove that the present sea, considered as to its entire quantity, is in a state completely stationary, so that the evaporation of its waters is equal to the quantity with which the rivers augment it, and its extent is neither diminished nor enlarged. But local circumstances, as for example the clearing of waste land, the destruction of forests, the choking up, or the turning away of the channels of rivers, may for a certain time alter the level of some interior seas. Other temporary or local causes may produce in the same ocean, not an augmentation or diminution of volume, but small oscillations, which, by subverting the equilibrium of the waters, occasion on one side the little retirings of the sea, consequently the formation of new ground; and, on the other side, little invasions of the sea upon the earth: these changes mutually compensate each other, and are limited, and too variable, to have any sensible influence upon the form of great continents. ces chan The present sea endeavours in two ways to change the BOOK. form of its banks: it creates new lands by depositing sand, XVIII. gravel, shells, and marine plants, by casting up and by re- Manner in taining the mud, and other substances brought down by which the the rivers; by undermining the mountains which border sea produit, which causes them at last to fall; by retiring of itself, ges. either because the rivers bring to it a diminished supply of water, or, because on another side, it has overflowed some ground over which it has spread a part of its waters. It has invaded the ancient shores, by washing them away, or by rising above their level, when from any cause whatever, its basin has been elsewhere confined. The seas of Europe, being the best known, will furnish the best proofs of our assertions. Let us begin with the Mediterranean. the eastern part of the ranean. We have seen, that by taking in Homer the word Changes in Egyptos for the name of the river, and not for that of the country, we may dispense with admitting, that the sea has Mediterfilled up the supposed ancient gulf, which penetrated into Egypt as far as Thebes, and which separated by one day's navigation the island of Pharos from the main land, as has been commonly affirmed. Some less considerable additions have, without doubt, been made to the land since the time when Herodotus gave us the first description of the country; but these are owing, less to the mud which the river carries down, than to the winds which bring with them the sands of the neighbouring deserts. This is what daily happens upon the coast of Egypt. The port of Alexandria is dammed up, the town of Damietta, whose walls, in the time of Louis IX. were washed by the sea, is now at a considerable distance from it. As a kind of compensation, the lake Mengalah appears to be formed, either by the overflowing of a branch of the Nile, the cleaning of which has been neglected, or by an irruption of the sea.f Along the eastern coasts of the Mediterranean and its gulfs, there is but little increase of the land. The island * See the description of Egypt in a subsequent volume. + Shaw's Travels, vol. ii. p. 173, 188. Telliamed, (De Maillet) sur la dimiaution de la mer, &c. BOOK of Tyre, however, has been united to the continent by a XVIII. more powerful hand than that of Alexander. At the mouth Changes in the Gulf of Venice. of the river Pyramus in Cilicia, a deposit of sand has extended the modern coast six miles beyond the ancient; and similar effects have taken place at other points on the southern shore of Asia Minor, where the coast is flat.* The Meander has, by little and little, filled up the valley into which it flows, and which was formerly a gulf. The inhabitants of Miletus, and of Ephesus, have several times changed the situation of their towns, by following the sea, which retired from their walls. On the coast of Greece, a great many small islands, situated in front of the openings of the rivers, are joined to the main land.t In the Gulf of Venice, very remarkable changes have taken place. Ramazzini having observed that all the country round Modena is suspended over a subterraneous lake, and that a great number of shells are to be found there, is persuaded that Lombardy has, in a great measure, been formed by the combined deposites of the Po, and of the sea. All that is certain is, that the Po in former times committed great ravages, by inundating often whole provinces: it has been confined by strong dikes, but by renewing from time to time these embankments, which preserve the country from total submersion, the bed of the river has been elevated, so that the level of the waters of the Po is now raised several feet above the lands which surround it. The environs of Ravenna, Aquileia, and Venice, present facts more uniform and better established. There is no doubt, according to the observations of Manfredi, that the grounds near Ravenna has sunk to such a degree, that the pavement of the cathedral is only six inches above the level of high water; but, at the same time, the land is extended in such a manner that this town, formerly situate in the midst of marshes, and canals, and furnished with an *Beaufort's Account of Caramania, p. 298. + Strabo, lib. ii. passim. Plin. Nat. Hist. ii. cap. 89. Manfredi, De Auct. Maris Altitudine, in Opusc. Bononiens, ii. 120. |