It is said that Alexander the Great was determined in his choice of the situation for the port of Alexandria by the consideration of the winds and littoral currents which carry eastward the matters held in suspension by the Nile, and thus cover the coast with sand. If this theory were true, no alluvium ought to be perceived westward of the mouth of the river. Now all the coast from Tripoli as far as El Aritch is covered with sand banks, which frequently form downs, and these downs are found at the present time transported several leagues into the interior of the lands westward of the Rosetta mouth. The port of Alexandria itself has not escaped the action of the ground swell, for a sand bank has been formed which occupies a good third of the total superfice of the roadstead. Happily for the port the accumulation of sand appears to have been arrested long ago, or rather its increase has become imperceptible. The roadstead of Alexandria owes its depth to the disposition of its sides in respect to the winds and currents. It is like the roadstead of Algiers, which is everywhere very deep, while the neighbouring ports have sand banks. It cannot be said that these sand banks are owing to the presence of rivers, which do not exist in the whole extent of the coast of Barbary, for the few land streams that are scattered along the shore cannot be called by that name. We have seen that the winds and the currents carry the detritus of the coasts reduced to sand to great distances. The currents however do not arrest the motion of the waves and the ground swell; they bend to their forms, and as their direction necessarily tends to the shore, the sands clear the currents with the ground swell which contains them, and which thus conducts them to the shore. When the direction of the waves is oblique to the coast, the sands are borne to a distance, but when it is perpendicular to the coast, the waves raise the sands brought by the ground swell into dykes and banks, which protect the low shores. The most minute and lightest portions are accumulated at the more elevated points of the flats, where, being dried by the sun, they are soon carried away by the wind, which leaves them, in its turn, in the shape of downs. The ground swell, therefore, furnishes the materials of those downs, which usually border flat shores, and it is that which has drawn from the depths of the sea the sands of those immense deserts of Africa, and of so many other plains which are found in various parts of the globe.