We halted for the night at Pang Pau, on the banks of the Meh Poi, which lies 2357 feet above the sea. This stream rises a few miles off to the north-west, not far from the gap through which the Huay Sai passes from Muang Hang. The Huay Sai, flowing to the Meh Ping, which empties into the Gulf of Siam, and the Meh Hang, which enters the Salween, flowing into the Indian Ocean, both rise in the same plain, which is only separated from the Meh Fang, which drains into the Cambodia river, flowing into the China Siam by the range we were about to cross. Next morning, after crossing the Meh Poi, we ascended a spur to the Pa Too Din (or Earthen Gate), the pass over Loi Kyoo Pa Săng. During the ascent it was raining heavily. The crest of the pass is 58¾ miles from Zimmé, and 2645 feet above the sea; and the hill is composed of a soft sandstone. Descending the slope for a quarter of a mile, we reached the bottom of the valley, which is said to be merely a long pocket in the hills, its drainage passing in underground passages beneath them. From the bottom we immediately commenced to ascend to the Pa Too Pa (or Stone Gate), which we reached after a toilsome climb of just one hour, the horizontal distance being barely half a mile. The slope was formed of hard blocks of traprock, with an outcrop of non-crystalline metamorphic rock. The path up the ravine was so steep and slippery in places that it seemed impossible for any animal less agile than a man to ascend it. Our elephants proceeded slowly but surely, keeping, like links in a chain, so close together, that one felt if one should slip he would carry the others with him. The path is not more than 18 inches broad, and is strewn with great rocks. It is said to be the most difficult pass in the country. Its crest lies 59½ miles from Zimmé, and 2916 feet above the sea. Mr Archer gives the altitude as 2750 feet above the sea; and Mr Gould, as 1600 feet above Zimmé. There may be a slight error in my height on account of atmospheric disturbance and a dense mist. Not far from the head of the pass, on the northern side, are two natural wells called Hoo Low, of great depth—one 6 feet and the other 10 feet in diameter. A pebble took four seconds in reaching the bottom. Two miles of easy descent among limestone hills brought us to the plain of Nong Vee-a, bounded on the north-west by a fine precipitous hill of mural limestone, called Loi Tum Tap Tow, rising about 1200 feet above the plain.