The four southern provinces which lie stretched east and west, to the south of the Yangtse basin and to the north of Tongking and the China Sea, are Yunnan, Kweichow, Kwangsi, and Kwangtung. The two latter are drained entirely by the West or Pearl River and its forks, which fall into the China Sea a short distance below Canton,—as are also portions of the two former provinces; Northern Yunnan and Kweichow, however, drain into the Upper Yangtse, while Southern Yunnan drains mainly into the Red River of Hanoi, flowing into the gulf of Tongking. In addition to these we find, traversing the southwest corner of the province of Yunnan, the Salwin, draining into the gulf of Martaban, and the Mékong with its delta in Cambodia. The four provinces are all mountainous, with little or no level land outside of the delta of the Pearl River, some high plateaux in Yunnan and a few narrow river bottoms. Yunnan is in fact a south-eastern peninsular extension of the great Tibetan plateau, itself a wide, similarly uneven, highland region in the nature of a plateau from six to seven thousand feet above sea-level, sloping gradually to the south and east, with ranges of mountains rising up to three and four thousand feet higher, with some peaks in the west above the snow-line (here fifteen thousand feet).