Korea is preëminently a mountainous country. With the exception of the alluvial plains at the mouths of the rivers, low ranges of mountains with narrow intervening valleys are found everywhere, and are characteristic. The main chain, forming the back-bone of the peninsula, is not clearly defined, as it is formed principally by the overlappings and intersections of minor chains, so that it is quite irregular as to direction, but a glance at the sources of the rivers, considered with reference to the intervening line of water-sheds, shows that it springs from the mountains of Siberia at the north, follows for some distance the line of the eastern coast and then strikes inland, trending to the southward and westward until it reaches the shores of the Yellow Sea. The loftiest ranges, therefore, are in the northern and eastern provinces. At the centre of the northern boundary is Paik-du-san, the "white-headed mountain," in whose slopes rise the Yalu, Tuman, and Songari rivers, the two former defining the western and eastern sections of the frontier, the latter a tributary of the Amur, an important stream of southern Siberia. According to Messrs. James, Younghusband, and Fulford, of the British Indian and Consular services, who visited it in May, 1886, Paik-du-san is "a recently extinct volcano with a lovely pellucid lake filling the bottom of the crater, surmounted by a serrated edge of peaks rising about 650 feet above the surface of the water. The height of the loftiest of these was found to be about 7,525 feet above the level of the sea." Besides the rivers of the frontier are others of the interior that deserve a passing mention. The mountainous nature of the country, as well as its proximity to the sea, implies the existence of numerous secondary water courses, but these as a rule are insignificant in size and so shallow as to permit of navigation only throughout limited portions of their extent. Among the larger streams that lie wholly within the country is the Taidong, flowing through Phyöng-an-do, the northwestern province, rising in the central ranges of the peninsula and flowing into the Yellow Sea. During the greater part of the year it is navigable as far as the city of Phyöngyang for native craft of the largest size. In midsummer its waters rise rapidly during a short rainy season; then quickly subside, the river resuming its former limits. To this sudden shoaling may be attributed the loss of the schooner Sherman, captured by the Koreans in 1871, the vessel going aground without warning at a place where a few hours before abundant water had been found.