This established, it has already been seen that the coast of the sea, as well as the projecting capes, resign to the sea every year a certain amount of earthy and rocky matters. These matters are removed by the waves which break upon the shore, the soft portions are quickly disintegrated by this powerful action, and form muddy sand and mud, and the hard portions are rounded into pebbles the size of which is diminished more and more by the prolonged action of the force which set them in motion and which reduces them to sand; but in proportion as these matters arrive at a sufficient degree of tenuity, they become susceptible of submitting to the transporting force of the waves and currents, and quit the place where they were formed. This transporting force depends both upon the height of the tides and the direction of the winds, as well as their intensity, combined with that of the currents which are observed in all seas. So that while considerable masses of matter are set in motion along the shores, the rivers, especially those which traverse a great extent of country, transport as far as their mouths only muddy matters, so light that they are carried to a distance, and afterwards deposited in the depths of the sea. This is remarkably the case with the Nile, whose waters at the time of the inundation are distinguished by their colour for more than ten leagues into the sea. All the deposits and accretions of the river up to 20 _kilom._ above its mouth are muddy, while all the banks which are at its mouth are composed of sand alone. Thus, all the impediments of the mouth of the Nile evidently emanate from the sea. To demonstrate it by still farther evidence, we will repeat the reasoning of the engineer M. Bonniceau relative to the alluvium of the river Mersey, in his excellent work upon the navigation of tidal rivers: “If the deposits emanated from the elevated lands in a sensible degree, the quantities deposited from time to time ought to be proportional to the quantity of rain that falls at the same epochs, because the same amount of matters descending from the elevated lands and transported by the river, ought to be partly regulated by the quantity of water that carries them; but it is a fact well ascertained, that the accumulations of sand which exist in the vicinity of the mouth are greater in proportion as the waters of the river are less abundant, while on the contrary at the time of the increase, when the Nile contains nearly 0 _met._ 008 of matter in suspension, the sand banks are removed and thrown back far off into the sea.”