Continental Monthly , Vol. 5, No. 6, June, 1864. VariousЧитать онлайн книгу.
the bar, in three fathoms of water, were raised above the surface by one of these banks, and were taken to the Balize, and used in building chimneys. In another instance, an anchor, which was lost from a vessel, was lifted out of the water, so that it was taken ashore. About twenty years ago, a sloop, used as a lighter, was lost outside the bar in a gale of wind; several years afterward she was raised by one of these strange formations, and her cargo was taken out of her.'
We may say the bumps of which Mr. Talbot speaks are termed 'mud bumps,' from the fact of being composed of sediment. They present a curious spectacle as seen from a passing steamer. They are undoubtedly the result of subterranean pressure, but from what cause, whether volcanic, or the influence of the sea or river, or both, has not been determined. Many speculations have been entered into in regard to these phenomena, but as yet without fruitful result.
Leaving this digression, we proceed to notice that the theories set up to explain the causes of the bars at the mouth of the river, have been numerous and various. Some suppose them to be the result of the water of the river meeting the opposing force of the Gulf waves, checking the current, and causing a precipitation of the suspended sediment. Others are of the opinion that the bars are entirely the effect of marine action, and endeavor to show that the immense inward flow of the Gulf washes up from its bed the vast accumulations that are continually forming in the way of navigation.
After a personal observation and investigation, and as well after frequent and free consultation with others, we are persuaded to discredit the above-mentioned theories. The resistance of the Gulf does not form the bars, though it exerts an influence. The immense volume and force of water ejected from the river receives no immediate repellent action from the Gulf, but extends into it many miles without the least signs of disturbance, as may be plainly discovered even in the most casual observation. It is known as well that the water of the river remains perfectly palatable at a very close proximity to the sea. This is a very good evidence of the superior force of the river's current. The two volumes of water mix a considerable distance out at sea.
An able engineer states that, upon examination, he found a column of fresh water seven feet deep and seven thousand feet wide, and discovered salt water at eight feet below the surface. As the result of his investigations, he divides the water into three strata, as follows:
1. Fresh water, running out at the top with a velocity of three miles an hour.
2. Salt water, beneath the fresh, also running out at about the same velocity.
3. A reflex flow of salt water, running in slowly at the bottom.
It is this inward current, he thinks, that produces the deposit, and in doing so carries with it no small degree of sea drift. The influx of the lower column flowing up stream, after it passes the dead point, is allowed time and opportunity for the sediment to deposit. The principle of the reflex current is somewhat that of an eddy, not only produced by the conflict of two opposing bodies of water, but also is much influenced in the under currents by the multitude of estuaries presented by the irregular sea front of the coast.
A gentleman, who seems to have taken a very statistical view of these bars, makes the following business-like and curious calculation as to their immensity: we introduce it on account of its originality. He says the average quantity of water discharged per second is five hundred and ten thousand cubic feet. The quantity of salt suspended, one in three thousand by volume. The quantity of mud discharged, one hundred and seventy cubic feet per second. Considering seventeen cubic feet equal to one ton, the daily discharge of mud is eight hundred and sixty-four thousand tons, and would require a fleet of seventeen hundred and twenty-eight ships, of five hundred tons each, to transport the average daily discharge. And to lift this immense quantity of matter, it would require about seven hundred and seventy-one dredging machines, sixteen horse power, with a capacity of labor amounting to one hundred and forty tons, working eight hours.
Another class of sedimentary formations met with along the banks of the Mississippi are the battures. There is one remarkable instance of these in front of New Orleans, which has led to much private dispute, and even public disturbance, as to ownership. Within sixty years, in front of the Second Municipality of the city, the amount of alluvial formations susceptible of private ownership were worth over five millions of dollars, that is, nearly one hundred thousand dollars per annum, and the causes which have produced them are still at work, and will probably remain so. As far back as 1847 these remarks were made upon the subject: 'The value of the annual alluvial deposits in front of the Second Municipality now is not less than two hundred thousand dollars, and, with the exception of the batture between the Faubourg St. Mary line and Lacourse street, all belongs to this municipality.' 'Such a source of wealth was never possessed by any city before. In truth, it may be said that nature is our taxgatherer, levying by her immutable laws tribute from the banks of rivers and from the summits of mountains thousands of miles distant to enrich, improve, and adorn our favored city.' There are numerous other examples of the kind going on elsewhere along the river.
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