The sand-bars just referred to frequently cross the mouths of rivers, and in such instances a struggle ensues between the currents moving along the shore and the outflowing river-waters aided by the currents produced by the tides. This conflict leads to the formation of sand banks and bars, generally submerged, across the entrances of bays and inlets and to the building of sand-spits from the seaward capes. A typical instance is furnished at the entrance of Mobile Bay (Fig. 7), where a spit from each side has been built by shore currents so as to greatly contract the tideway between. Similar features are presented by Sandy Hook and Coney Island, each of which has been built of sand deposited by shore currents at the seaward entrance of the lower New York Bay. Another illustration of this same general character is furnished by the curved extremity of Cape Cod (Fig. 8), which is a sand-spit of large size with a hooked extremity. Spits of this nature are common on our coasts, and in many instances themselves form harbours, as at Coney Island and near the extremity of Cape Cod. Many other similar examples of the importance of lagoons, sand-bars, spits, etc., to shipping, which occur, especially along the Atlantic coast of the United States, may be studied to advantage on the admirable charts of the United States Coast and Geodetic Survey. [Illustration: FIG. 8.--Cape Cod, Massachusetts.] The sand-bars, spits, and other similar structures along the Atlantic coast are also of strategic importance, for the reason that they afford advantageous sites for fortifications, as is illustrated by the strong forts at Sandy Hook which guard the entrance of New York Bay. These sea-built foundations are also utilized in a large number of localities for lighthouses. The waterways shut off from the sea by off-shore bars in some instances permit of the passage of vessels from one harbour to another. In this connection it is of interest to note that an important system of canals is under consideration for making a continuous waterway for deep-draft vessels, some 700 miles long, which will connect the estuaries and lagoons from New York to the Carolinas.