Geological Survey 7th Annual Report Obsidian Cliff, Yellowstone National Park

The tourist who, in his wanderings in search of entertainment, adventure, or health, has been so fortunate as to visit the great National Park vividly recalls how, on the first day's ride from the Mammoth Hot Springs to the geyser basins, after traveling up the picturesque road along Glen Creek, at the northern base of Mount Bunsen, to Rustic Falls, he came upon Swan Lake Valley and beheld for the first time the grand and imposing peaks of the Gallatin Mountains, which extend from Electric Peak, 11,000 feet in height, southward to Mount Holmes, with an altitude of 10,300 feet After driving over open country to the crossing of the Gardiner River and through the flat-bottomed valley known as Willow Park, he entered the timber and rode along the west bank of a narrow stream, which fights its way through grass and fallen trees, until he reached the northern end and outlet of Beaver Lake. On his left, to the east, stretches a long, low cliff, the southern end of which is formed of nearly vertical columns of black obsidian, or volcanic glass, which has resulted from the rapid cooling of a perfectly fused, igneous rock. From this great blocks have fallen and accumulated at its base in a talus slope, over which has been built what is popularly known as the glass road, the material of which it is made being as true a glass as any artificially produced. The colors and structure of this natural glass not only make it the most interesting rock the visitor will find, but the phenomena of its occurrence in this locality are of special scientific importance, and the present paper has been prepared with the view of describing and explaining it.