effected by means of irrigation from the Frontier river, the exact identity of the source of a river, if this be mentioned in a Treaty or Convention, or of its main affluent, or, in a deltaic region, of its mouth, the provision required for navigation, police, and fiscal control—all of these suggest possible difficulties in the acceptance of a river boundary, particularly in new or tropical countries, which cannot be ignored. In ancient and civilized States the procedure to be followed in many of these cases is regulated by international agreement or by the Law of Nations. The general principles regarding the navigation of rivers traversing different States were indeed embodied in Articles 108–116 of the Final Act of the Congress of Vienna, and have been applied by subsequent Agreements to some of the principal rivers of Europe, Africa, and America.
The last Natural Frontier to which I need here refer is the wellnigh obsolete barrier created by forests and marshes and swamps. The various Saxon kingdoms of England were, for the most part, thus severed from each other. When Caesar first landed in Britain, the head quarters of Cassivelaunus, the British leader, were placed at Verulamium, near St. Albans, which was surrounded by forests and swamps. Arminius gained his famous victory over the Roman legions by entangling them in the forests and morasses of Westphalia, and there is no more poignant picture in history than the description of Tacitus of those mournful scenes. Venice was many times saved from absorption both by foreign invaders, such as Goths and Huns, and by jealous neighbours by her cincture of lagoons.