Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/292

Rh and long lagoons, but eventually breaking through the barriers of alluvium and entering the sea. The lagoons of the Italian coast and of the Baltic near Dautzic are familiar examples. A conspicuous series of these alluvial bars fronts the American mainland for many hundred miles round the Gulf of Mexico a11d the shores of 1’lorida, Georgia, and North Carolina. A. space of several hundred miles on the east coast of India is similarly bordered. M. E. de Beaumont, indeed, has estimated that about a third of the whole of the coast-lines of the continents is fringed with such alluvial bars. (_f.) Deltas in the Sea.——The tendency of sediment to accumulate in a tongue of ﬂat land when a river loses itself in a lake is exhibited on a far vaster scale where the great rivers of the continents enter the sea. It was to one of these maritime accumulations, that of the Nile, that the Greeks gave the name Delta, from its resemblance to their letter A, with the apex pointing up the river, and the base fronting the sea. This shape being the common one in all such alluvial deposits at river months, the term delta has come to be always applied to them. A delta therefore consists of successive layers of detritus, brought down from the land and spread out in the sea at the mouth of a river until they reach the surface, and then, partly by growth of vegetation and partly by ﬂooding of the river, form a plain, of which the inner and higher portion comes eventually to be above the reach of the floods. Large quantities of drift- wood are often carried down, and bodies of animals are swept off to be buried in the delta, or even to be ﬂoated out to sea. Hence, in deposits formed at the mouths of rivers, we may always expect to ﬁnd terrestrial organic remains. Vhen a river enters upon the delt-a—portion of its course it assumes a new character. In the previous parts of its journey it is always being augmented by tributaries ; but now it begins to split up into branches, which wind to aml fro through the ﬂat alluvial land, often coalescing and thus enclosing insular spaces of all dimensions. The feeble current, no longer able to bear along all its weight of sedi- ment, allows much of it to sink to the bottom and to gather over the tracts which are from time to time sub- merged. Hence many of the channels get choked n p, while others are opened out in the plain, to be in turn abandoned, and thus the river restlessly shifts its channels. The sea- ward ends of at least the main channels grow outwards by the constant accumulation of detritus pushed into the sea, unless this growth chances to be checked by any marine current sweeping past the delta. The European rivers furnish many excellent illustra- tions of delta—growth. Thus the Rhine, Meuse, Sambre, Seheldt, and other rivers have formed the wide mari- time plain of Holland and_the Netherlands. The Rhone has deposited an important delta in the Mediterranean Sea. The upper reaches of the Adriatic Sea are being rapidly shallowed and filled 11p by the Po, Adige, and other streams. Thus Ravenna, originally built in a lagoon like Venice, is now 4 miles from the sea. The port of Adria, so well known in ancient times as to have given its name to the Adriatic, is now 14 miles inland, while in other parts of that coast-line the breadth of land gained within the last 1800 years has been as much as 20 miles. On the opposite side, also, of the Italian peninsula, great additions have been made to the coast—line within the historical period. It is computed that the Tuscan rivers lay down as much as 12 million cubic yards of sediment every year within the marshes of the Maremma. The “yellow” Tiber, as it was aptly termed by the Romans, owes its colour to the abundance of the sediment which it carries to sea. It has long been adding to the coast-line at its mouth at the rate of from 12 to 13 G 110 L O G Y [nu D'.'.MICAT.. feet per annum. Hence the ancient harbour of Ostia is now more than 3 miles inland. Its ruins are at present (1879) being excavated, but evcryllood of the river leaves a thick deposit of mud on the streets and on the floors of the un- covered houses. Whcnce it would seem that the Tiber has not only advanced its coast-line, but has raised its bed on the plains by the deposit of alluvium_, so that it now over- flows places which, 2000 years ago, could not have been so frequently under water.‘ In the Black Sea a great delta is rapidly growing a.t the mouths of the Danube. At the Kilia outlets the water is sh-allowing so fast that the lines of soundings of (3 feet and 30 feet are advancing into the sea at the rate of between 300 and -100 feet per annum.‘-’ The typical delta of the Nile has a seaward border 1I*'0 miles in length, the distance frmn which to the apex of the plain where the river bifurcates is 90 miles. That of the Mississippi contains an area of 40,000 square miles. The united delta of the Gauges and Bralnnaputra covers a space of between 50,000 and 60,000 square miles, and has been bored through to a depth of 481 feet. (‘I/.) ;9'ea-borne ;S'eclinze72t.—Although more properly to be noticed under the section on the sea, the final course of the materials worn by rains and rivers from the surface of the land may be referred to here. By far the larger part of these materials sinks to the bottom close to the land. It is only the ﬁne nmd carried in suspension in the water which is carried out to sea, the distance depending on the velocity of the stream, the specific gravity and shape of the particles of the mud, and the help or hindrance given by marine currents. The sea fronting the Amazon is dis- coloured for 300 miles by the mud of that river. The soundings taken by the "Challenger" brought up land- derived detritus from depths of 1500 fathonis,—several hundreds of miles distant from the nearest shores. The amount of material carried by a river into the sea. may be taken as the measure or gauge of the general lower- ing of the surface of the basin drained by the river. If we ascertain the annual quantity of mineral matter thus de- livered into thesea, and know the superﬁcial extentof ground from which it has been derived, the one sum divided by the other gives the extent by which the mean level of the country is reduced in one year. Both the ﬁne mud mechanically suspended in the water and the salts dissolved in it should be taken into account, as well as the coarser detritus pushed along the bottom. It is the mechanically suspended mud which has generally been measured. According to the data of Messrs Humphreys and Abbot, already cited, the propor- tion of sediment in the Mississippi is 33163 by weight, or by volume. The annual discharge of sediment is 7,-l59,267,200 cubic feet, and the drainage basin 1,147,000 square miles. This is equal to a loss of -65%, of a foot of rock from the general surface of the drainage basin in 1 year, or 1 foot in 6000 years. Other rivers work faster than this rate. The Ganges has been estimated to remove 1 foot of rock from its drainage area in 2358 years, and. the Po 1 foot in 729 years. Such computa- tions are at the best only approximations to the truth, but they are useful in showing how great an amount of change must be effected even -within comparatively short geological periods by the] vayious agents which are disintegrating the surface of the an. 1 See an interesting article by Professor Charles llartins on the Aigues- Mortes, in Revue dcs I)eu:z: Jlfondcs, 1874, p. 780. The present writer accompanied the distinguished French geologist on the occasion of his visit to Ostia in the spring of 1873, and was much struck with the proofs of the rapidity of deposit in favourable situations. _In the article just cited some valuable information is given regarding the progress of the delta of the Rhone in the Mediterranean. 2 lI:u-tlcy, Min. of Proc. Inst. Civ. L'n_r/z'u., xxxvi. 216.