OCEANICE RIVER SYSTEMS The foregoing articles on the distribution, relative areas and position, principal and minor divisions, of the great waters of the globe, together with their intercommunication, require a brief sketch of the principal oceanic river-systems, to com plete what may be termed the general geography of the oceans. The principal general facts have been thus grouped together in the first part of the work as being absolutely necessary to further study of the physical and special geography of the oceans.
We have said (art. 2) that the ocean covers the more considerable depressions of the earth's outer crust, and that the land, generally speaking, is more or less elevated above the surface of the sea. Water naturally seeks to reach the lowest possible level, and thus all, or nearly all, the running waters of the globe flow directly or indirectly into the ocean ; the only exceptions being a few rivers which discharge their waters into inland seas or lakes, or are lost in arid deserts, their volume of water being too limited to withstand the double action of excessive evaporation and constant infiltration through the generally permeable ground over which they flow. The ocean, then, being the great receptacle of the rivers of the globe, it follows that the natural classification of the various river-systems must correspond to the natural divisions of the sea. The land masses being contiguous to four of the five oceans, we have primarily four great oceanic river-systems, the Arctic, Atlantic, Indian, and Pacific. Leaving the details of each system for insertion in the sections devoted to each ocean, we shall now simply make a general classification, noting only the principal rivers.
The universal tendency of all running waters to move towards the sea is, as we have said, due to the relief or comparative elevation of the land masses generally. What particular ocean or part of the ocean they will flow into depends likewise upon the relief of the particular portion of the land over which they flow. It being a fact of almost
universal application, that the more elevated portions of the land proximate more or less to the ocean, the relief of all the land masses will, generally speaking, present the same general characteristics; the main element being, on one side, a short steep counter-slope towards that part of the ocean immediately approximating it, and a long slope on the other. The latter, of course, facilitates the formation of rivers of considerable length ; the former, of shorter and more rapid streams.
Applying the above principles to the continental land masses, we find their most perfect and uniform illustration in the western mountain ranges of America, which follow the coast line of the Pacific Ocean from Alaska on the north, to Tierra del Fuego on the south, with remarkable regularity. The Andes in South America uniformly follow the sinuosities of the coast, while the Rocky Mountains in North America preserve the same parallelism to the coast, though at a greater and more variable distance. In the Old World the working out of the same law of position is seen in the mountain chains of southern Europe approximating generally to the Mediterranean—the Himalaya, Nanling, and other ranges of South-eastern Asia approximating to the Indian Ocean and China Sea; while even the central ranges of the Kwen-lung, Pe-ling, Thian-shan, approach nearer the Indian than the Arctic Ocean. In Africa, again, and in Australia, all the great mountain chains are situated on or near the sea ; in either case following the general trend of the coast, e.g. the Bong, Nieuveldt, and Drakensberg Ranges in the former, and the Australian Alps, Blue Mountains, and Liverpool Range in the latter. In our own islands it is scarcely necessary to contrast the elevated districts on the west, and the more level tracts on the east, of England. A careful examination of good physical maps will enable the student to make a complete list of cases illustrating the law of position of mountain ranges.