ALLUVIUM 339 subside near the shores and build up the allu- vial 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 exist- ence of the same animals that now inhabit the region. What the rivers are accomplishing in the ulterior, the tidal currents are effecting along the coasts. They wear down what has been built up in former times, and strew the mate- rials in new deposits of alluvium. In Ger- many these accessions, called Anlandung, are of great value along the coast of the North sea. On the American coasts they are more com- monly 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 super- ficial covering of the rocks of Long Island is nothing more than the accumulations of sedi- ment discharged by the Hudson, Hackensack, Passaic, and Earitan 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 occa- sionally 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 be- came 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 invis- ible 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 exam- ination 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 se- crete 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 pre- vious epochs have in the present period. And the same remark may be extended to peat, beds of which are found rivalling, in the quan- tity of carbonaceous matter they contain, the beds of fossil fuel, into which they too will in time be converted. The most interesting fea- ture 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 rec- ords 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 charac- terize 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 mate- rials of our houses and household utensils. The sands are used for many purposes in the me- chanical 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 scat- tered materials of the ore together, and as it evaporates leaves them in form suitable for use. As the ores are removed, more collect and re- new 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 pos- sessed by organic bodies : a belief which, after
