Although but just emerging from a high, snowy mountain, the Yellowstone is here a bold, rapid, and deep stream, one hundred and twenty yards in width. They continued their course along the river till the 23d, when the party embarked on board of two canoes, each of which was twenty-eight feet long, sixteen or eighteen inches deep, and from sixteen to twenty-four inches wide. Sergt. Prior, with two men, was directed to take the horses to the Mandans for safe keeping until the re-union of the expedition. July 24.--At eight o'clock, Capt. Clarke and the remainder of his party embarked, and proceeded very steadily down the river. They passed the mouths of several large rivers emptying into the Yellowstone; one of which was called the Big-horn, from the numbers of that remarkable species of sheep seen in its neighborhood. Next day, Capt. Clarke landed to examine a curious rock, situated in an extensive bottom on the right, about two hundred and fifty paces from the shore. It is nearly two hundred paces in circumference, two hundred feet high, and accessible only from the north-east; the other sides consisting of perpendicular cliffs, of a light-colored, gritty stone. The soil on the summit is five or six feet deep, of a good quality, and covered with short grass. From this height, the eye ranges over a wide extent of variegated country. On the south-west are the Rocky Mountains, covered with snow; on the north, a lower range, called the Little Wolf Mountains. The low grounds of the river extend nearly six miles to the southward, when they rise into plains, reaching to the mountains. The north side of the river is bounded by jutting, romantic cliffs, beyond which the plains are open and extensive, and the whole country enlivened by herds of buffaloes, elks, and wolves. After enjoying the prospect from this rock, to which Capt. Clarke gave the name of Pompey's Pillar, he descended, and continued his route. At the distance of six or seven miles, he stopped to secure two bighorns, which had been shot from the boat, and, while on shore, saw in the face of the cliff, about twenty feet above the water, a fragment of the rib of a fish, three feet long, and nearly three inches round, embedded in the rock itself. BEAVERS, BUFFALOES, MOSQUITOES. The beavers were in great numbers along the banks of the river, and through the night were flapping their tails in the water round the boats.