Near its mouth the Upper Yellowstone is about half the size of the main stream as it leaves the lake. Its valley is about three miles wide and very marshy; all the little streams flowing down from the wooded hill-slopes being obstructed by beaver-dams, so as to form continuous chains of ponds. The sides of the valley are dark, sombre walls of volcanic rock, which weathers into curious and imposing forms. Looking up the valley from some high point, one almost imagines himself in the presence of the ruins of some gigantic city, so much like ancient castles and cathedrals do these rocks appear--a deception that is not a little heightened by the singular vertical furrows cut deep into the cliffs. At the base of the walls immense masses of breccia have fallen from the mountain tops, in many instances cutting long swaths through the pine forests. In the upper part of the valley, which in midsummer is lush with vegetation, five streams flow down from the mountains to swell the waters of the Yellowstone. These streams Colonel Barlow calls, in honor of his commander's greatest victory, the Five Forks. Here the valley terminates abruptly, the mountains rising like walls and shutting off the country beyond. Just at the head of the valley is a little lake, a hundred yards or so in width; the large lake which has been placed on maps as Bridger's Lake having no existence. Dr. Hayden with two assistants ascended the mountains to the west of the head of the valley to survey the district bordering on the great divide. From this point as far as the eye can reach on every side are bare, bald peaks, domes and ridges in great numbers. At least one hundred peaks worthy of a name can be located within the radius of vision. The rocks everywhere, though massive, black, and deeply furrowed vertically, have the appearance of horizontal stratification. In some instances the furrows are so regular that the breccia has a columnar appearance. The summits of the mountains are composed entirely of breccia, containing angular masses of trachyte, from 10 to 30 feet in diameter, though most of the fragments are small. Dr. Hayden's party camped at night near a small lake, by the side of a bank of snow, 10,000 feet above the sea, with short spring grass and flowers all around them. There are but two seasons on these mountain summits, spring and winter; as late as August fresh new grass may be seen springing up where a huge bank of snow has just disappeared. Little spring-flowers, seldom more than two or three inches high, cover the ground--_Clatonia_, _Viola_, _Ranunculus_, and many others. The following morning they travelled for several miles along a ridge not more than two hundred yards wide, from one side of which the waters flow into the Pacific, and on the other, into the Atlantic. To the westward the outlines of the Teton Range, with its shark-teeth summits, are most clearly visible, covered with snow. From whatever point of view, the sharp-pointed peaks of this range have