The argument set forth in the last chapter is simple, ingenious, and persuasive. Nay, we must fairly confess that to us it seems conclusive. We do not mean to say that it amounts to a rigorous demonstration. But it affords at least a strong presumption that the process of deposition, the process of consolidation, and the process of stratification, are going on to a vast extent beneath the waters of the ocean; and that, in these latter ages of the world’s history, Aqueous Rocks are slowly growing up under the influence of natural causes, which resemble in every important feature those that are now attracting so much attention within the Crust of the Earth. We are therefore prepared to accept this conclusion, if it be not found at variance with any well-established fact, or with any known and certain truth. But in matters of physical science the evidence of our senses is, after all, the most satisfactory argument. And our readers, no doubt, would like to witness, if possible, with their eyes, the building up of Stratified Rocks. Now, though it is not given to us to see this process in all its colossal magnitude as it goes on within the depths of the mighty ocean, it is yet possible to behold it exhibited, as it were, in miniature, in certain cases where the sediment of rivers is deposited within reach of observation.
Every one is familiar with the fact that many rivers overflow their banks at certain seasons, and spread themselves out over a wide area, sometimes reaching to the foot of the hills that bound the valleys through which they flow. This is the origin of those Alluvial Plains so remarkable for their surpassing richness and fertility. In each successive year a thin film of sediment is deposited on the surface of the land; and thus in the course of ages a soil is formed capable of producing, season after season, the most luxuriant crops without manifesting any symptoms of exhaustion. The soil of the Alluvial Plain near St. Louis, on the Mississippi, is thus spoken of by a modern traveller: “As to the quality of the land, any given number of crops might be grown off it. Corn has been raised on it for a hundred years together—as far back as the settlement is known. To inquire about the system of farming in the West is not productive of information which would be of service on the continent of Europe. There is no system: the farmer scratches the ground and throws in the seed, and his bountiful harvests come up year after year without further thought or trouble. Thousands of centuries have made the soil for him, and it defies him to make too heavy demands upon it. It gives him all he asks, and is never known to disappoint or fail.”[42]
The great basin of the Nile offers an admirable example of an Alluvial Plain on a scale of considerable magnitude. Even in the days of Herodotus, Egypt was regarded as the “gift of the Nile:” and the correctness of this opinion has been placed beyond all reasonable doubt by the investigations of modern science. The river bears along in its current, especially during the flood season, a large quantity of fine earthy sediment obtained by the process of Denudation from the mountains of central Africa. Once a year, between the months of July and November, it overflows its banks, and this sediment is deposited on the adjoining plains. Thus a new layer of rich soil is spread out every year over the existing surface; and the whole country is, in a manner, growing upward at the average rate, according to a rough estimate, of about six inches in the century. Near Cairo, where excavations have been made, the successive layers of annual deposit are distinctly visible to the eye. And it is worthy of remark that, although each one of these is no thicker than a sheet of paste-board, the stratum of alluvial soil which overlies the sands of the desert, and which to all appearance has come into existence by the very same process, is often forty, fifty, and even sixty feet in depth.
A series of interesting observations and experiments have been recently made under the auspices of the Royal Society, which afford some useful information on this subject. The colossal statue of Rameses, near Memphis, was found to be partly embedded in a stratum of mud which had gradually accumulated around it. Upon sinking a shaft, it was discovered that from the present surface of the plain to the base of the pedestal is a distance of nearly ten feet. Now, Rameses flourished, according to Lepsius, about one thousand three hundred and sixty years before the Christian Era; and therefore, since that time, or within a space of 3200 years, it is pretty clear that a thickness of ten feet has been added at this spot to the Alluvial Plain of the Nile. It is hard to resist the conclusion that the next stratum of ten feet as we proceed downward, which, in every respect, resembles the first, must have been produced in the same way by natural causes; and so on till we reach the barren sand of the desert, which is here just forty-two feet below the present level of the plain.[43]
It should seem, therefore, that Egypt is nothing more than a great Alluvial Plain, slowly built up in the long lapse of ages, by the annual inundations of the Nile. Vast tracts of the same kind are to be found in other parts of the world. The Mississippi, which drains about one-seventh of the whole North American continent, has formed an Alluvial Plain more than a thousand miles in length, and from thirty to eighty in breadth. And in South America, the Orinoco once a year spreads out its swollen and turbid waters over an area not unfrequently seventy miles broad; leaving behind, when it subsides, a substantial layer of muddy sediment to enrich the soil.[44] It would be easy to accumulate examples. But we shall be content with having referred the reader to the Great Basin of the Nile, which affords special opportunities for the study of alluvial phenomena; being illustrated at once by the historical monuments of remote antiquity and the scientific researches of recent times.
There is another process by which Alluvial Plains are formed. It often happens that a river fills up the channel in which it has been moving for years, and is forced to shift its course and seek a new passage to the sea. In progress of time this channel is filled up like the former and deserted, and then a third, and then a fourth. At each change a new stratum is formed, almost always distinguished for its extraordinary fertility. This phenomenon is chiefly to be looked for when an extensive and almost level plain lies between some lofty range of mountains and the sea. In such a case, the river which bears away the waste of the mountains, will move onward in its course with a sluggish current, and will, of necessity, deposit the greater part of its burden on the way. There is scarcely a country in the world that does not abound in formations of this kind; and we could point to many notable instances in which herds of cattle are now grazing on the very spot where, within quite recent times, the turbid waters of some great stream flowed sullenly along.
The river Po, which receives through a thousand mountain torrents an enormous quantity of mineral sediment from the Alps, affords an instructive example. Since the beginning of the fifteenth century it has many times changed its course, often committing great devastations, and always leaving behind unmistakable traces of its movements. Several towns that once stood on the left bank of the river are now on the right. In some instances parish churches and religious houses were pulled down when the devouring stream was seen slowly to approach, and then rebuilt with the same materials at a greater distance. An old channel may be easily recognized at the present day near Cremona, which bears the name of Po Morto, and another called Po Vecchio, in the territory of Parma.
It may be interesting to our readers to learn that these movements have been checked in modern times. By a system of artificial embankment the waters of the river are now confined within definite and narrow limits: thus the velocity of the current is increased and a very considerable portion of the sediment is carried on to the sea. Nevertheless, much is still deposited in the bed of the river, which is, in consequence, raised higher and higher each successive year. Hence it has become necessary, in order to prevent inundations, to add every season to the height of the embankments, so that the river now presents the appearance of an enormous aqueduct, of which some idea may be formed from the fact that, in the neighborhood of Ferrara, the surface of the stream is higher than the roofs of the houses. This system of embankment is carried on very extensively in Northern Italy to check the overflowing of rivers, and to prevent them from changing their courses. It is as old as the time of Dante, who tells us that the inhabitants of Padua erected barriers along the Brenta when the snows began to melt and the season of the floods was approaching,
“Per difender lor ville e lor castelli,
Anzi che Chiarentana il caldo senta.”