Some miles above Vien Chan, the Mekong enters a narrow valley, which is sharply defined and enclosed by two ranges of high hills. Its waters, hitherto majestic and tranquil, which had peacefully unfolded silver coil after coil over the vast plateau of central Laos, now accelerated their course, and tumbled and eddied among the rocks, ever restless and ever noisy. The noble river, which had previously measured its breadth by thousands of yards, now shut up within two barriers of constantly-increasing elevation, was now contained in a channel which rarely attained to five or six hundred yards in width, and from which it was no more to escape. In dry seasons it occupied only a small portion of this space, and it had presented a rugged and broken surface of rock; a grand mosaic, where fragments mingled of all the metamorphic formations--marbles, schists, serpentines, even jades,--curiously coloured, and sometimes admirably polished. [Illustration: PASSAGE OF A RAPID.] As the travellers advanced the river grew narrower, and, with a width of three hundred yards and a depth of twenty-five fathoms, flowed through a wild and wooded valley, uninhabited except by the animals of the forest. They passed the mouth of the Nam Thon; after which they came upon a dangerous series of rapids, where the foaming waters, hurled and driven from side to side, and swung round projecting rocks, and driven against the foot of precipitous banks, rushed downwards tumultuously, with all the clang and clash of billows breaking against a reef. To thread this water-labyrinth, it was necessary to obtain the assistance of a pilot from a neighbouring village; and even he was unwilling to promise that the boats of the expedition, light and small as they were, could be carried up to the next Muong, that of Xieng Cang. The boats, however, were unloaded, and the stores transferred to the shoulders of sturdy natives, who bore them along the rocks; while others towed the boats with many a lusty pull through the whirl and foam of the rapids. But so laborious and so difficult was the task, that two whole days were spent in effecting the passage of a few miles. ~AT MUONG MAI.~ ~A CENTRE OF TRADE.~