rivers of the first magnitude, is by no means universal; on the contrary, some of them terminate in deep inlets, or Estuaries, instead of projecting forms: or, if the expression may be allowed, they terminate negatively, instead of positively. Of this class may be reckoned the great rivers of the Amazons, Plata, and the Oronoko; besides many others, which perhaps bring down an equal quantity of the matter of alluvion, with the Nile, the Ganges, or any other river, that may form the most projecting Delta. This difference appears to be owing to the original conformation of the adjacent coast, and to the depth of the sea beyond it. If the Estuarium into which the river discharges itself, and the sea beyond it, are exceedingly deep, the alluvial matter will be lost in the profundity; whilst in a shallower sea, not only the bed of the inlet itself will be filled up, but the matter will form a projecting tract beyond it. And here it may be observed, that the increase of Deltas will almost necessarily be slower in modern than in ancient times; since the farther the work advances the deeper the space to be filled up must be.

The Nile is amongst that class of rivers which has the most remarkable, and most prominent Deltas: and its Delta, from the celebrity of the country, of which it forms so considerable a part, has been the theme of history, from the earliest times. Accordingly, we are enabled to trace many of its changes, from positive records; whilst those of other rivers can only be traced from the appearances which they exhibit.

Before we endeavour to trace these changes, it

will be proper to offer some general observations on the courses of rivers, through their own alluvions ; on the original formation of Deltas, composed of such alluvions; and on their subsequent changes.

All Deltas, as would appear by the sections of the river banks, as well as of the ground itself, to a great depth, are formed of matter, totally different from that of which the adjacent country consists; proving that they are the creation of the rivers themselves; which rivers, having brought down with their floods, vast quantities of mud and sand from the upper lands, deposit them in the lowest place, the sea; at whose margin the current, which has hitherto impelled them, ceasing, they are deposited by the mere action of gravity.

It is no less certain, that during the progress of forming by its depositions, the low land which is to constitute the future delta, the river, by its overflowings above, also raises such parts of the adjacent countries, as are subject to be overflowed by its waters. And hence it must be conceived, that such rivers must gradually raise their beds: since, in order to run at all, they must have a continued declivity, the whole way to the sea: so that the very act of extending their course, by forming new land in the sea, requires a gradual elevation of the ground the whole way from the margin of the sea, upwards. Thus, alluvial countries must continue to rise, by slow degrees, whilst the alluvions encroach on the sea; and the rivers themselves continue to overflow and deposit.

The declivity, or slope, of the new formed land,

as well as of the old, will be regulated by the influence of the level of the sea, on that of the floods of the river for although the river may swell 30, or more feet, with the periodical rainy season, in the parts removed from the sea, yet at the point of its junction with the sea, it cannot rise at all; since water cannot be retained in a heap, but must form a common level with the mass with which it mixes. The land flood will therefore form a slope of such a nature, as its gravity, combined with the declivity of the stream, will admit: and it appears from experiment, in another river, (the Ganges) that the slope commences about the head of the Delta. But in the Nile, we are told that it begins much higher; which is very probable, as its Delta is so much smaller than that of the Ganges. Below the point in question, at any given place, the elevation of the periodical flood, as well as the level of the country, bears a pretty just proportion to its distance. from the sea. This matter is abundantly proved by experiment, and may be verified with ease.

But as the Delta of the Nile, in common with other tracts of the same nature, was founded in the sea; and, in consequence, the course of the river itself must have been prolonged through a tract, which cannot, in the nature of things, be formed (notwithstanding the regular and constant depositions of the floods) into so great a slope as that part of its bed, which lies through the original land; it must of necessity happen, that there will be two different degrees of slope, in the beds of such rivers the steepest over the original land; and the

13

least steep, over what was originally the bed of the

sea.

This opinion seems to receive confirmation from the history of the river before us; although other instances of the like kind could be adduced. For, it will appear, that the head of the Delta of the Nile has absolutely moved downwards, several miles, since the date of history: which must, doubtless, be owing in part to the extension of the greater slope downwards by the depositions of the floods; if we admit them to have raised the original land every where equally, and to have formed in a course of time, a stratum of vast depth; by which operation, the angle formed by the termination of the greater, and the commencement of the lesser, slope, is, in effect, removed downwards. These two slopes may be compared to the slope of a hill, and that of a gently declining plain, at the foot of it. A stream will run down the hill, in a channel nearly straight, but having reached the plain, it wanders, and separates into different branches. If the hill could be removed within the edge of the plain, the place where the windings and separation began, would advance in the same proportion: and thus we regard the two slopes, and their operation.

It appears quite unnecessary to offer any arguments in proof of the assertion, that alluvial countries gradually rise; or that they gradually encroach on the sea; since the sea coasts of all Deltas project beyond the general line of the coast: that islands in the sea, have in several instances been joined to the main land, by the matter deposited by rivers: and

that not only history, but ocular demonstration teaches us, that the levels of different alluvial tracts, are very considerably raised. But, it is probable that the progress of the elevation, as well as of the encroachment, has been very much over-rated, in many instances; and in none more than in the case of the Nile, by modern travellers. That the Delta has increased in the part towards the sea, since the days of Herodotus, cannot be questioned; when the increase of such coasts, in other countries, are perceptible to the senses. The quantity of the increase, in a given time, is, however, a desideratum: for it happens that the record of the distance of the sea coast from Heliopolis, (in Herodotus) on which an argument has been founded, is quite erroneous 3. It

Pliny has a catalogue of islands that have been thus joined to the main land; lib. ii. c. 85, 89. More will be said concerning the elevation of the soil in the sequel.

3 Our Author says, Euterpe 7, that the distance from Heliopolis to the sea, differs only 15 stadia, from that betwixt Athens and Pisa: 1500 stadia being the exact distance betwixt Heliopolis and the sea. Now, it has appeared in p. 21, vol. i. that Athens and Pisa are distant from each other, 105 G. miles; but Heliopolis is no more at this present time than 88 miles from the Canopic mouth of the Nile, which was probably the part meant (for the sea is, at present, much nearer in some directions); and from the supposed point of the Delta opposite to Heliopolis, 86; which was probably the place reckoned from. Hence, it may be clearly perceived, that no comparison can be drawn between the present, and the former extent of the Delta; since the number of the stades is wrong in the first instance. (See the reasonings of M. Savary and M. Volney on this fact, in their respective books of Travels.) But M. Volney has, however, made some excellent observations on the Nile, and its inundations and alluvions.