Many lowland plains rise so slightly above the sea level, that they are not unfrequently submerged, and, in many cases, owe their existence to repeated overflows. They are the basins of old gulfs, as in the very slightly elevated plains of Caracas, whose whole shore is open to the influences of the great Atlantic current flowing from east to west; or, as in the great Lombardy plain, which slopes at the same almost imperceptible degree toward the Adriatic. There are also some lowlands found in the interior of continents, and these, too, sinking below the level of the sea; but they are altogether exceptional, and only met with in two or three instances. They are called, by an accommodation of an algebraic term, _negative_ lowlands. To them belong the region around the Caspian and the Aral Seas, and the much smaller tract comprising the Dead Sea, and forming the Jordan valley; besides, there is the Suez steppe, inclosing the bitter lakes between Asia and Africa; and possibly the Beled-el-Jereed, in the western part of the Sahara and the central part of Australia. To these it might not be incorrect to join those partial lowlands which have been rescued by human efforts from the sea; the marshes, for instance, behind the dikes of Holland, Sleswick, East Friesland, and at the mouths of the Vistula, the Weser, the Nile, the Ganges, and other rivers. The most extensive lowlands in the world are probably those which embrace Siberia, in Asia, and the Canadian and polar region of North America. Many great tracts, entirely inland, are in those flat districts covered by sea-water which was once driven in by great storms, and now lies stagnant, resulting in inapproachable swamps and morasses. Yet, under the equator, there are immense lowlands, as, for instance, in the eastern Sahara, although this region is broken by strips of plateau, and is by no means that uniform lowland plain which it used to be regarded. Northern Australia belongs to the same category, and also those immense plains which reach from the Atlantic so far into the interior of Brazil, along the lower Amazon. By the time, however, that they reach the middle course of that river, they have acquired, though in such imperceptible steps, a considerable degree of elevation, according to Humboldt’s barometrical observations, and not reckoning certain limestone hills found there, from 1050 to 1200 feet. The plains of the middle Marañon are, therefore, true plains, but not absolute lowlands, and not to be identified with the great flat region at the mouth of the river, and in comparison with the real lowlands of Venezuela, which do not rise over 200 feet above the sea, and genuine plateau, which, level as it is and broad as it is, is far more elevated than the Valdai plateau, in Russia.