Alluvium (Lat., from alluere, to wash upon or against), the deposits of sand, gravel, marl, etc, brought down by running streams of the present geological period. Other recent accumulations also, as those of peat and of the hills of sand blown together by the wind, are often called alluvial. They all belong to Lyell's uppermost group, the post-tertiary, and are characterized by containing human relics and remains. The same group comprises the calcareous rocks of recent origin which occur on the coast of Guadeloupe, and contain human skeletons imbedded in solid limestone, and also the coral reefs which are in process of formation in tropical seas, spreading out in calcareous strata hundreds of miles in extent. These are not usually included in the term alluvium; and yet it is not easy to draw a line that shall exclude any formations of recent origin; for the wash of rivers, as it settles in the bays at their mouths, often finds some cementing matter that soon binds it into solid rock, and in this hard rock are entombed as fossils works of art or remains of man. Thus the term alluvium has no precise signification.

The great deposits of alluvium accumulate so slowly and silently, that we little appreciate the immense changes made by running water upon the surface of the earth; yet in the short period from the time to which our records extend back, we find that the sediments of a few small Italian rivers have carried out the coast line into the gulf of Venice from 2 to 20 miles; and that the ancient port of Adria, which in the time of Augustus gave its name to the gulf, is now an inland town, 15 miles from the shore. According to Herodotus, the ancient priests of Egypt regarded their country as " the gift of the Nile." From the great pyramids down to the sea all is made land. The great rivers of the world, as the Mississippi, Amazon, Ganges, and Orinoco, are producing effects far greater than those of the Nile; but our observations of these extend but a few generations back, and we lack sufficient data for calculating very exactly the rate of increase of their deltas. With the Mississippi, however, this has been attempted by Mr. Forshey, an eminent engineer, from observations extending through 30 years.

Adopting the estimate of Dr. Riddell of New Orleans, that the weight of sediment is 1/1245 that of the water, or 1/3000 of its volume, and allowing the quantity of water brought down per second to be 447,199 cubic feet, the whole amount of sedimentary matter annually added to the delta and carried into the gulf is equal to 4,083,333,333 cubic feet, enough to cover 144 square miles one foot deep. And yet at this rate, for the river to have built up the great accumulations of alluvium which make its delta, would have required 61,000 years; and higher up there are the accumulations at this rate of some 30,000 years more. Thus long at least, it is probable, the great rivers have flowed as they now flow; and during this latest epoch few changes have occurred in the lower forms of animal life; for in the strata next older than these alluvial deposits, the land and river shells are all of the same species with those now living in the same region. Subsequent investigations by Capt. Humphreys and Lieut. Abbot (1858) have given results which will be described under Mississippi River. - The delta of the Ganges and Bramapootra is far more extensive than that of the Mississippi. It is a wilderness filled with a labyrinth of rivers and creeks, infested with tigers and crocodiles, and larger than the principality of Wales. The rivers pour down their turbid waters charged with sadiment, and abounding with the ruins of animal and vegetable life.

These are swept into the bay of Bengal, the waters of which are discolored by the fine mud nearly 100 miles from its mouth, while the heavier materials subside near the shores and build up the alluvial strata. Near Calcutta, it was ascertained on boring for water that these strata continue below the surface to the depth reached, which was 481 feet. They were alternations of beds of clay and of marl, with others of decayed vegetable matter like peat, which last no doubt had at times formed the surface, until submerged by subsidence, and then buried beneath the deposits of the rivers. In these strata various fragments of fossil bones and shells were brought up, all of which indicated the existence of the same animals that now inhabit the region. - What the rivers are accomplishing in the interior, the tidal currents are effecting along the coasts. They wear down what has been built up in former times, and strew the materials in new deposits of alluvium. In Germany these accessions, called Anlandung, are of great value along the coast of the North sea. On the American coasts they are more commonly of a sandy character, stretching out in long beaches, the material of which is blown inland by the winds and piled into barren hills.

The long sandy strip of land, called the Great South Beach on the S. side of Long Island, which is a remarkable example of these sandy strips or "spits," is more than 100 miles in length, exceeding any such accumulation in Europe. These sands are now formed into alluvial beds by the action of the winds and of the ocean currents; but there is good reason to believe that the greater proportion of the superficial covering of the rocks of Long Island is nothing more than the accumulations of sediment discharged by the Hudson, Hackensack, Passaic, and Raritan rivers. - Alluvial deposits are frequently found in positions above the level of present running waters. Thus, around the shores of some of our great lakes are occasionally seen in the banks layers of sand and clay containing the same species of shells that are now common in their waters, but several feet above their reach. It was during this modern period of the formation of the alluvium that the gigantic mammoths and mastodons became extinct. Their bones are found in the peat bogs and marl beds, the origin of which probably does not extend far back from the introduction of man. Indeed, if we may place confidence in the traditions of the American aborigines, we must believe that these animals were contemporary with man.

Within the ribs of a mastodon found in Warren county, N. J., in 1845, were seven bushels of vegetable matter. In the western states the bones of these animals are generally discovered in the low places around salt licks which are still frequented by the deer and other wild animals that come to suck up the saline waters. - If the alluvium is interesting for these gigantic fossils, it is no less so for the microscopic forms of vegetable and animal life, which, though invisible to the eye, yet by the immensity of their numbers exceed in aggregate bulk that of all the mastodons and mammoths that have ever lived. The silicious deposit, resembling fine white marl, found underlying peat, and at the bottoms of ponds and marshes, especially in a region of primary rocks - a substance often used as a polishing powder - is found on examination by the microscope to consist of the remains of diatoms and desmids. - As these vegetables secrete from the primary rock its principal ingredient, so testaceous animals secrete from the limestone the calcareous matter for their shelly coverings; and of their remains are made up the marl beds and other beds of alluvium that abound, in shells, as the oyster banks and muscle beds of our coast.

The lime of which the latter is composed is no doubt mostly abstracted from that held in solution in sea water. But salt water, fresh water, and land shells all flourish best where limestone rocks abound; and where this source of lime is deficient, they even acquire the materials of their own shells from the remains of former individuals. The accumulations of this nature going on in our ponds, lakes, and harbors, though now little apparent to observation, are a part of the alluvial formation that will have an important bearing in the future economy of our globe, as the similar formations of previous epochs have in the present period. And the same remark may be extended to peat, beds of which are found rivalling, in the quantity of carbonaceous matter they contain, the beds of fossil fuel, into which they too will in time be converted. - The most interesting feature of the alluvium, which has been already incidentally alluded to, is its being the only geological formation which contains human relics and remains.

In no other formations are they found, or ever will be; for the races of animals and plants that have lived at different periods have not failed to leave permanent records of their most delicate organizations, and in the rocks of a very different epoch are still to be seen the footmarks Left by strange forms of birds. Thus man and his works characterize the rocks of this period, as the gigantic birds characterize the new red sandstone, and the great saurians the formations from the lias to the chalk. - The alluvial deposits produce our most fertile lands. The clays are the materials of our houses and household utensils. The sands are used for many purposes in the mechanical arts. Bog iron ores collect in low marshy places from the filtration of water through older formations, in which ferruginous matters of various forms are diffused. The water dissolves the oxide of iron and conveys it away, as it dissolves the potash from ashes through which it leaches. It gathers the scattered materials of the ore together, and as it evaporates leaves them in form suitable for use.

As the ores are removed, more collect and renew the supply; so that they are believed by many, who do not comprehend the manner of their silent accumulation, to be endowed with a principle of growth analogous to that possessed by organic bodies: a belief which, after all, may not in one sense be so very extravagant; for according to the researches of Ehrenberg, the ochreous particles, under the microscope, prove to be portions of an organic body of extreme minuteness, which is now believed to be a plant.