The Rhine, in the higher part of its course, is filling np the Lake of Constance, where n considerable tract of alluvial land has been formed ; and, after issuing pure from the lower end, it. appears from the observations of Hammer to have carried on the work of destruc tion so powerfully in the comparatively short distance between the Lake of Constance and the bottom of the falls at Schaffhausen, as to have supplied materials sufficient to fill up several lakes between Schaffhausen and Strasburg, besides the great Lake below Strasburg already spoken of. There are numerous instances of this gradual filling up of lakes, especially in the courses of the greater rivers, as in the Danube between Ulm and Nenburg above Vienna, and most eminently so in the case of the St-Lawrence. Simoud states that the river Lint, in Switzerland, is perpetually filling up its old channel, and overflowing into a new one, in consequence of the mass of rubble!' and stones brought down from the Marna Mountains ; and that the level of the Lake of Wellenstrult had been actually raised 10 feet in the previous CO years by this accumulation. If the river does not meet with lakcs in its course, and flows over a great extent of country having a slight degree of inclination, the transported matter very often so accumulates as to raise the bed of the stream itself. One of the most striking instances of this kind is afforded by the I'o, the common receptacle of the waters of the numberless torrents which rush down on both *Idea of It, loaded with spoils from the Alps and Northern Apennines. The effect of this has been that the river Ilan frequently shifted its course ; and, to prevent the damage that ensues from such events, the inhabitants of Lombardy have protected their lands by embanktnente, which confine the river to its channel. This, however, is a work of incessant labour, and deceptive security, for the accumulation of matter in the bed goes on with unremitting constancy ; and, to prevent the water from overflowing, the matter must be taken from the bottom and thrown upon the banks, sometimes as much as a foot in n season. The effect of this has been, that in the lower parts of its course the Po runs on the top of a high mound, which even overtop' the houses in Ferrara.
In a mountainous country where the land rises rapidly from the shore, the rivers descending over a steep bed sweep all the contents into the sea. If the neighbouring sea be deep, and the tides be strong, an estuary or inlet is formed at the mouth of the river—that. is, the sea forms a deep indentation into the land, of a triangular shape, forming what Rennell and other geographers have fancifully called a ' negative If, on the other lured, a low shelving shore, and the absence of 'drum; tidal currents favour the gradual and tranquil deposit of the solid matter brought down by the river, an extensive level of alluvial land is formed. In this case the main river, at a distant point inland, often divides itself into two streams, which, gradually diverging until they reach the sea, ineloso a triangular space of land liming the form of the fourth letter of the Greek alphabet, A, and hence called a (Mkt. The mass of water does not, however, long continue divided into two streams only ; the process of separation is repeated several times, and thus the delta is traversed by several channels, and the great river empties itself into the sea by many mouth& as may be seen by the inspection of the Nile and Ganges in any map of Egypt or Hindustan on a tolerably large scale. In this way a delta is formed at the mouths of the Rhine, Rhine, Po, Danube, \Volga, Nile, Indus, Ganges, Orinoco, and many others. The magnitude of the delta, genendly, although not always, corresponds to the volume of the voters by which it has been created. The head of that of the Rhine
is about 110 miles distant from the general lino of seacoast of Holland ; and although the vane of the main river is almost lost by the sub division of its waters and the junction of other rivers, we include within the Rhino delta the whole of the low-land from the neighbonr hood of Calais to the north-eastern snores of the %nyder Zee, which makes the base of the triangle nearly 200 miless. The heel of the delta of the Ganges is 220 miles from the sea, its base is 200 miles long, including the space occupied by the two great arms of the Gauges which bound it on either side. The tract in the lower part of this delta, called the Sunderbunde, a wilderness infested by tigers and croco diles, is, according to 'Hornell, equal in extent to the principality of Wales. The whole of a deposit within a delta, as well as much above and on each side of it, is therefore an encroachment of the land upon the sea, and in many riven; this growth of the land is in a steady pro gress of advancement ; as, for example, the city of Ravenna, formerly a seaport of the Adriatic, is now 4 miles inland. There are causes, however, which often prevent the farther increase of a delta after it has advanced a certain length such seems to be the case with the delta of the Nile, which does not advance with the rapidity that might be expected from thequantity of matter brought down by the river. [NILE, in ECM. Div.) Great as is the amount of new land thus formed, it is insignificant in comparison with the quantity of solid matter carried down by rivers, and deposited in the depths of the WA. It is impossible to form any estimate of this upon which reliance can be placed, because no accurate observatione have been made to supply the data. To come to anything like a satisfactory conclusion, it would be to have a vertical section of the river at a given point, obtained by numerous entindinge, so as to get the profile of the bed, and by observations nt different season's to get the mean height ; we must also have the results of expe riments throughout the year, to ascertain the mean velocity, and the volume of solid matter contained in a given bulk of the water. The quantity of mud and sand poured by the Ganges into the Ray of Bengal is so great, in the flood-season, that the sea recovers its tram pareney only at the distance of 60 miles from the coast. Sir Charles Lyell, in his 'Principles of Geology,' makes a calculation (founded upon the computations of Major Rennell) as to the mean quantity of water discharged by the Ganges into the sea, by which he shows that, sup posing the water to contain one hundredth pert of solid matter, a masa equal in bulk to the greatest of the Pyramid's of Egypt is brought down by the Ganges every day. The sea is discoloured for many leagues from the mouths of the Orinoco, and the solid contents, swept by ocean currents through the Gulf of Paris, after being partly deposited on the shores of Guiana and the island of Trinidad, are carried into the Carib bean Sea and Gulf of Mexico. By the observations of Colonel Sabine, it appears that the muddy waters of the river Amazonas may be distin guished 800 miles from its mouth. The great basin of the Amazonas, which is drained by that mightiest of rivers and its vast and countless tributaries, embraces an area, according to Humboldt, only one-sixth less than the whole of Europe, and through this the main stream flows for nearly mil's. The river, nt the point where its waters unite with the of the Atlantie, according to the same illumtriona traveller, 90 nines brond.