When once the underlying principles on which depend the characteristics of coastal topography are suggested, any observant person can apply them for himself and thus be able to read the history as well as admire the beauties of seacoast scenery. It is not necessary, therefore, to attempt to present a detailed account of the coasts of North America from a purely geographical point of view; there are certain results of the processes just referred to, however, which are of wide-reaching economic, and especially of commercial interest. [Illustration: FIG. 6.--A portion of the Atlantic coast of the United States.] From Central America northward to Cape Cod the rocks bordering the sea are soft or easily soluble, and the adjacent land of low relief. Throughout this section the work of the sea is mostly constructive, and the margin of the land is sheltered by sand-bars from the attack of waves and currents. Where the waves of the open ocean do reach the land, as on the coast of New Jersey, the sea-cliffs are low and the topography of a mild type. Very generally, as along the coast of Mexico and Texas, and from Florida to Long Island, there are long narrow bars adjacent to the shore, with lagoons intervening between them and the mainland. These features are well illustrated on the accompanying map (Fig. 6) of a portion of the Atlantic coast where long narrow bars, sometimes forming skeleton capes, are a characteristic feature. On the middle Atlantic coast of the United States the prevailing winds blow southward and there is a general southward flow of the shore currents, which carry with them the sand on the beaches and bars. An interesting fact in this connection, pointed out by N. S. Shaler, is that although the sands are continually being moved they are not worn out. After the sand-grains have been reduced to a certain size they retain films of water which separate them one from another, and act as cushions which prevent the grains from coming in contact, thus greatly retarding further comminution. But for the protection thus afforded the sand-bars would be removed and the border of the land exposed to the attack of the waves and cut away; whereas under existing conditions lagoons are formed, which in many instances are utilized as harbours or are filled by wind-blown sand, the sediment brought by streams, plant growths, etc., and valuable additions are made to the continent. [Illustration: FIG. 7.--Mobile Bay.]