The second great river of importance is the Bang-Pa Kong, which has its source in a barrier range of irregular mountains, separating the elevated plateau of Korat from the alluvial plains extending to the head of the Gulf of Siam. The river meanders through the extensive paddy-lands and richly cultivated districts of the northeast provinces, and falls into the sea twenty miles east of the Meinam. Another considerable river is the Meklong, which falls into the sea about the same distance to the west of Bangkok; at its mouth is a large and thriving village of the same name. This is the great rice district, and from Meklong all up the river to Kanburi a large number of the population are Chinese. In this valley are salt-pits, on which the whole kingdom depends for its supply. The Meklong is connected with the Meinam by means of a canal, which affords a short cut to Bangkok, avoiding the sea passage. A third river system, that of the Mekong, much the largest of all the rivers in Indo-China, drains the extreme north and east of Siam. This huge stream, which is also mentioned in Camoens' Lusiad, takes its rise near the sources of the Yangtse Kiang in Eastern Thibet, and belongs in nearly half its course to China. It was partly explored by M. Mouhot, and later (in 1868) by Lagrée's expedition, who found it, in spite of the great body of water, impracticable for navigation. M. de Carné, one of the exploration party, thus sums up the results of the search for a new trade route into Southern China: "The difficulties the river offers begin at first, starting from the Cambodian frontier, and they are very serious, if not insurmountable. If it were attempted to use steam on this part of the Mekong the return would be most dangerous. At Khong an absolutely impassable barrier, as things are, stands in the way. Between Khong and Bassac the waters are unbroken and deep, but the channel is again obstructed a short distance from the latter. From the mouth of the river Ubone the Mekong is nothing more than an impetuous torrent, whose waters rush along a channel more than a hundred yards deep by hardly sixty across. Steamers can never plough the Mekong as they do the Amazon or the Mississippi, and Saigon can never be united to the western provinces of China by this immense water-way, whose waters make it mighty indeed, but which seems after all to be a work unfinished."