ALLUVIUM, or ALLUVION. A. term which, in the English language, has no very defined meaning. Some authors use it to designate all those rafts which have been formed by causes now acting on the surface of the earth, including those of volcanic origin; while others, adhering to the literal meaning of the original term, con fine its application to 4eposits, whatever be their character, that have resulted from inundations. Neither of these definitious convey the same meaning as is usually attached to the word, the one including too much, the other too little. The term is now used in Its proper application to designate all those deposits recently formed, or now forming, by the agency of water, whether from an uninterrupted and constant stream, or from casual inundations. All streams, lakes, rivers, seas, and the ocean itself, hold a large quantity of earthly matter in mechanical solu tion, which they deposit in their beds. The character of the sediment is governed by the na ture of the rocks over which the waters flow; and the quantity depends partly upon the constitution of the rooks, and partly upon the power of the water. If the rock be easily destroyed, and a large body of water flow over it with a considerable velo city, the destructive effect will be great, and much worn materials (detritus) being formed, the stream will have a thick and turbid appearance. The same effect is frequently produced by the discharge of a number of tributary streams into a river, all of which accumulate a greater or less quantity of the earths over which they flow. The distribution of water at the present time, more particularly refering to rivers, is very differ ent from that of periods. The majority of the valleys through which rivers are new flow ing, have been produced by the action of water. which, running from higher lands, has not only scooped them out, but has spread over them the worn material which it accumulates in its pas sage. By the operations which have since been going on, the waters have been collected to gether in comparatively narrow channels of con siderable permanency. On this account, the influence of water that over the portions of the earth inhabited. by terrestrial animals is greatly restricted; and the production of new beds of rock or soil is rather an accidental than a necessary consequence. But, although the influ ence of water has been thus confined, all lands, and especially the surfaces of mountainous dis tricts, are undergoing change; and the superficial covering on one district is conveyed to, another. The showers of heaven are constantly sweeping away the soil and decomposed rocks 9f the up lands into the valleys, over which they are trans ported by streams and rivers, the larger and heavier particles falling to,the bottom, the smaller being united with the water in mechanical mix ture. That portion of earthy matter which is carried away from a district by the running water is, as far as the district itself is concerned, the most valuable, being the superficial covering or soil, and would be forever lost to that portion+ of the earth inhabited' by man, were it not arrested in its passage to the ocean, by deposition in the bed of the river, or on those lands which the waters may happen to overflow. It is well known to those who have visited elevated dis tricts, that many mountains are already deprived of then soils, and are but the skeletons of the earth, without covering or life. By this action the val leys are in the process of elevation, and the mount • sins of depression; and if we could conceive it to proceed without limitation, we may imagine a time when all the varieties of elevation and depression, which now give beauty to the sur face, will be destroyed, and an entirely different condition of the distribution of land and water will be established. But, at the same time, it can not be denied that these changes, as far as. they have hitherto proceeded, have been advan tageous to man, whatever might be their result under the conditions to which we have alluded. The mountainous regions are, from their eleva tion, less suited to the progression of society, so intimately connected with agricultural prosper ity, than the plains. As we rise above the level of the sea, the atmosphere becomes more rarefied, and the cold more intense, both of which are in jurious to vegetation in general, and unsuited to promote the comfort of animal life. The plains are therefore, preferred by men when they con gregate together and form societies. It can not be considered an unwise or unfit result, that the lowlands, should be enriched with alluvial soils, produced by the destruction of the rocks and natural soils of mountainous regions. It is re ported of Dioclesian, that he told his colleague,, Maximilian, he had more pleasure in the cul tivation of a few pot-herbs which, . in the gar den of Spalatro, grew in the soil that on the top of Mount Hzemus had only produced moss and dittany, than in all the honors the Roman empire could confer. From the definition here given of the word "alluvium," it must include the gravels and sands that are of recent formation among the alluvial deposits; but our attention is chiefly directed to the soils, or those beds which are suited to sustain vegetable life. It is true that the ,gravels may be made available for the cultivation of some plants, but the beds which are so used belong rather to' that class of rocks de nominated diluvial by geologists, than to the deposits of which we are speaking. If we trace the circumstances under which alluvial soils are formed to their cause, we shall find that they have their origin in the fall of heavy rains, and the of snows, in mountainous regions. The water, in its passage to the valleys, collects the superficial soil and decomposed earthy ma terial that lies in its path, and transports them into the channels towards which it flows. The streams that are formed on the mountain slopes are generally united together before they reach the plains, and form impetuous torrents, over coming all obstacles, until their velocity is lost, when, in their winding- courses, they meet each other and form rivers. Rivers, in every part of their course, are subject to inundation; when, throwing their waters over a considerable space, they deposit the earthy materials they have accumulated. If such inundations had not occurred, the accumulated worn materials (debris) would have been deposited in the bed of the river, or carried into the lake or sea where the waters themselves are discharged,. There are abundant instances on record of the filling up of rivers by the worn materials (detritus) which have been carried into their courses, and the rivers of our own country will afford excellent examples of this result. Many rivers and estu aries, in some portions of the globe, which a few years since were navigable, have ceased to be so on account of the large amount of alluvial matter de posited in their beds; and many towns and cities which were once populous and wealthy, have on this account become poor and almost deserted. If we would see the effect of the transport of worn materials into lakes, we can not have a more favorable opportunity than in Switzerland. Many of the lakes of this sublime and majestic country are rapidly filling from this cause; and in some of them water plants are seen above the surface of the water. But when a river suffers inunda tion, the earthy matter, which is held in mechan ical mixture, is arrested, and deposited on the land that is overflowed, and a richly productive soil is formed. One or two examples may illus trate these remarks. The Ganges annually over flows its banks, and a rich alluvial soil over the country it inundates. This magnificent river was supposed to take its rise on the northern side of the Himalaya mountains, until it was proved, in 1819, by Lieutenant Webb, that all the streams which unite to give its existence, take their rise on the south side of the Hindoo Coast, or Snowy mountains. The melting of the snows, and the heavy periodical rains augment' the volume of the water, and by the end of June, before the rainy season has commenced in the low country, the river has generally risen fifteen feet; but after the rains in Bengal it usually attains a height of thirty-two feet above its ordi nary level. By the end of July all the low countries adjoining the Ganges and the Burram pooter are overflowed, and nothing but houses and trees are seen for many miles inward. The province of Bengal is divided into two nearly equal parts by the Ganges; and as a large portion of•the country on the banks of the river is low, it is especially exposed to inundation, froth which circumstance it probably derives its name, such districts being called bang. A deep bed of rich soil is deposited during the period of the over flow, and the vegetable productions are of the most varied and luxuriant character. Rice, Wheat, barley, tobacco, indigo, 'otton, the mul berry, and the poppy, are all oultivated with success on the alluvial soils. It is well known that Egypt has been, from time immemorial, in- • debted to the overflow of the Nile for a rich alluvitil soil, as well as for the means of irrigat ing the land. The ancients seem to have been
altogether at a loss to account for the periodical overflow of this river;, and when we consider the appearances before them', we are not surprised at the difficulties they experienced. They ob served it in a country that was not moistened by a drop of rain,, and when it was unaided by a single stream, and yet, at its stated period, it began to lift its waters from their bed, and rising higher and higher, overflowed its banks, and spread 'itself like a sea over Lower Egypt, re freshing the parched earth with moisture, and aiding its produCtiveness with the formation of a superficial covering of rich loam. The philoso phers speculated without success upon its cause; but while they were disputing as to the origin of the phenomenon, year by year the Nile rose, and left the evidence of its beneficial sway in the richness of the crops and the luxuriance of the country. From the investigations that have now been made, we know that the rise of the Nile is occasioned by the rains which fall on the high mountains in the interior and tropical regions, and not, as many of the ancients sup posed, from the Etesian winds, which, blowing periodically from the north, prevent the waters from reaching the sea. The great importance of rivers, as agents in the, production of alluvial soils, can not be more strongly proved by any positive evidence .than, by a Consideration of the state of Australia, a country remarkable for the fewness of its rivers, and the general poverty of its soil. Contrary to all precedents, the richest soils in this land, excepting the alluvial, are found on the summits of hills. The fires which so frequently happen on the plains, the peculiar character of the vegetation (chiefly consisting of evergreens),, and the sparing distribution of water, are the principal causes of the sterility of this otherwise desirable country. There are, however, spots which, covered with alluvial soil, can rival the richest and most cultivated dis tricts of England; and the comparison of these with other lands impresses the observer the more strongly with the great importance of the natural provision for the restitution of that portion of the earth inhabited by man, by the disposition of new earthy matter and a virgin soil. The alluvial flats of the Nepean, the Hawksbury, and the Hunter rivers, are spoken of by all writers as remarkable for their fertility. The rich valley in which the Lake Alexandrina is situated may be noticed as another example of the influence of alluvial soils. The country around this lake appears to be one of the most beautiful and fertile in Australia; and a glance at the map will immediately inform the inquirer of the cause. It is so situated as to receive the worn materials of the mountain chain that ranges along the prom ontory of which Cape Jervis ift the southern point, and also to obtain moisture from the lake, and a renovating soil whenever it may overflow its banks. Alluvial soils are produced by the discharge of mountain streams into valleys, as well as by the overflow of rivers. We have already explained the manner in which they col lect the superficial covering of mountainous districts, and being charged with earthy matter, bring it into the plains. This may be deposited before the streams are united together in an individual channel as well as after, and should this be done, the valley may he cohered with alluvial products. The formation of a river is a process which requires time, and many changes must happen before the flowing waters can form for themselves a local habitation ; obstacles must be removed, a bed must he scooped out, and an outlet must be formed, in the performance of which earthy matter must he accumulated, and extensive deposits be formed. A third cause in the production of alluvial deposits may be men tioned. The sea is making great inroads upon many of its shores, carrying a destructive war against the 'cliffs that vainly indeavor to oppose its force; while on the other hand it' is in some instances receding from the shores against which it once beat; and thus,. as though to recompense man for what it takes away, gives to him a por tion of its own territory. Those districts which are thus added to the land are usually superposed by a fine, rich, alluvial soil, as also are those which have at a former period been covered by the sea, and would be at the present day, were it not for the ingenuity and works of man. The districts in which are situated New Orleans, in America, and Missolonghi, in Greece, are chiefly alluvial, and nearly the whole of Holland has the same character, and can only be described as a district of which man has robbed the ocean. That part of the coast of Germany which is bor dered by the Nolth Sea is alluvial, and additions are constantly made to the shores by the gradual deposition of earthy matter upon the immense flats which extend along them. The first sign of vegetation on these lands is the _appearance of the saltwort (Salicornia maritima), which is succeeded by the sea grass (Pen marikma), and when the land is very rich, by the marsh star wort (Aster Tripolium). The land is afterwards dyked, and used as pasture for sheep and cattle; so that the spot over which the sea has perhaps for ages exercised an undisputable control, is brought under the pow& of man in a state most admirably adapted to suit his wants. But it may be asked, whence does the sea obtain the earthy matter with which it abounds? Rivers discharge themselves into the ocean, and it has been already stated that their waters are charged, more or less, with superficial soil of mountainous countries and the destroyed materials of rocks. A part of this may be arrested by occasional or periodical inundations, and by deposition in the bed of the river, but a large quantity must still be carried into The ocean. It must also be remembered that the waters which is conveyed in a channel is constantly endeavoring so to arrange its course so as to suffer the least possi ble resistance. In this attempt, it attacks the banks that confine it, and widens its course, pre cipitating much earthy matter into the stream, to be removed by the flowing water. It fre quently happens, and especially after the fall of heaVy rains, that the water at the mouths of rivers is thick and turbid from the quantity \of alluvial matter it holds in solution, and very many large rivers are rendered unsafe for naviga tion by the existence of large bars of sand or clay at their outlet. But the sea is not merely a passive recipient, of the product of destructive causes, but is itself a cause. Sea coasts are con stantly suffering depredation by the action of the waves that beat upon them. Whether we look at the Soft and almost unresisting rocks of the pastern coast of England, or ta .e hard primary rocks of Devonshire, Cornwall, and the Shetland Isles, the same result will be observed. During the stormy months of winter, the waves are tossed upon the coasts with an almost uncon trolled violence, no rock is sufficiently hard to resist its energy, and when unruffled by a passing breeze in the months of summer, its influence upon the softer rocks is hardly less destructive, though more insidious, for it then attacks the base of the cliffs, and removing the support of the superincumbent mass, causes the precipita tion of large portions into the sea. By these two cause the sea is provided with the materials for the formation of alluvial soils. Some estimate may be formed of the violence and extent of these causes, by an exaraination of the present state of the German Ocean, one fifth of which is covered by banks that appear to have been pro duced In the same way as the alluvial soils on the northern coast of Germany. Water, then, is a most powerful agent in the destruction and production of rocks, and were there no conserva tive principle, the changes that are going on would be more extensive than they are in the present day. The floods to which some rivers are subject are so impetuous that they frequently sweep away all opposing and involve an entire district in ruin. These effects, however, are much more common in countries that are thinly covered by vegetation than in those where it is luxuriant, for it acts as a conservative agent, increasing the power of the resistance by binding the soil more closely together. This, therefore, will account for ,the diminished influence of floods upon low lands, and for the frequent deposition of rich and fertile alluvial soils. The composition of the, alluvial soils that have been brought under cultivation is exceedingly various; but they are generally remarkable for their fertility, and are admirably suited for pasture lands. In general, says Sir Humphrey Davy, the soils, the materials of which are most vari ous and heterogeneous, are those called alluvial, or which have been formed by the deposition of rivers; many of them are extremely fertile.— Appleton's New Cycloptedia.