The abrasion of the coasts of the channel supplies the sea with fragments of chalk and siliceous rocks, which being rolled about by the sea become shingle. This shingle forms banks along the English and French coasts, and forced by the double action of flood and wind towards the straits it approaches them; but the shingle on the coast of France continually decreasing in size, reaches the mouth of the Somme, where it finds the point of Cayeux formed by its accumulation. Stopped at this point by the waters of the Somme, and by the change in the direction of the current of the sea which turns towards the Pas de Calais, this shingle increases the point of Cayeux, so long as its continual collision has not sufficiently reduced the size of the stones for them to be carried away by the sea; but when they are small enough, the flood bears them away and distributes them on the numerous banks which are found between the Somme and the Pas de Calais. From the inspection of Marine Charts, it is seen that the fineness of the deposit increases in proportion as these banks are nearer to the straits, and if the banks disappear in the Straits, it is because the force of the current does not allow the sands, which from being sifted for a long time have become finer and finer, to stop in that passage. They pass it therefore and some go to form the downs between Dunkirk and the Scheld, others in like manner to form downs on the English coast, others remaining in the strongest currents are carried as far as the mouths of the Humber in England, and of the Meuse and Rhine on the Continent. If the shape of the English and French coasts to the north and south of the Straits is observed attentively, it will strike every one that those to the south are cut out into concave indentations, while those to the north all affect the convex form. It is because the coasts to the south of the Straits are abraded by the tide, and those to the north, on the contrary, are fed by the accretions. As for the muddy matters in this long course, they can only be deposited in a few perfectly tranquil creeks, or in the basins of open ports on either coast. Wherever the tide penetrates they are carried with it, and, when finally it has entered the northern sea, and made a course sufficiently long to abate its swiftness, it finds itself in an excellent condition for depositing these muddy matters, which it holds in suspension. This is what it does at the mouth of the Humber, where it completely chokes up the port of Hull.