"Meanwhile the party in the canoes advanced slowly up the river till they came to a large island, to which they gave the name of Three-thousand-mile Island, on account of its being at that distance from the mouth of the Missouri." FOOTNOTE: [2] There are many stories, from other sources, confirmatory of these noises in mountainous districts. One solution, suggested by Humboldt,--who does not, however, record the fact as of his own observation,--is, that "this curious phenomenon announces a disengagement of hydrogen, produced by a bed of coal in a state of combustion." This solution is applicable only to mountains which contain coal, unless chemical changes in other minerals might be supposed capable of producing a similar effect. CHAPTER VIII. THE SOURCES OF THE MISSOURI AND COLUMBIA. Aug. 12, 1805.--Capt. Lewis decided to advance along the foot of the mountains, hoping to find a road leading across them. At the distance of four miles from his camp, he found a large, plain, Indian road, which entered the valley from the north-east. Following this road towards the south-west, the valley, for the first five miles, continued in the same direction; then the main stream turned abruptly to the west, through a narrow bottom between the mountains. We traced the stream, which gradually became smaller, till, two miles farther up, it had so diminished, that one of the men, in a fit of enthusiasm, with one foot on each side of the rivulet, thanked God that he had lived to bestride the Missouri. Four miles from thence, we came to the spot where, from the foot of a mountain, issues the remotest water of the mighty river. "We had now traced the Missouri to its source, which had never before been seen by civilized man; and as we quenched our thirst at the pure and icy fountain, and stretched ourselves by the brink of the little rivulet which yielded its distant and modest tribute to the parent ocean, we felt rewarded for all our labors. "We left reluctantly this interesting spot, and, pursuing the Indian road, arrived at the top of a ridge, from whence we saw high mountains, partially covered with snow, still to the west of us. The ridge on which we stood formed, apparently, the dividing-line between the waters of the Pacific and Atlantic Oceans. We followed a descent much steeper than that on the eastern side, and, at the distance of three-quarters of a mile, reached a handsome, bold creek of cold, clear water, running to the westward. We stopped for a moment, to taste, for the first time, the waters of the Columbia; and then followed the road across hills and valleys, till we found a spring, and a sufficient quantity of dry willow-brush for fuel; and there halted for the night." THEY MEET WITH INDIANS.