On the Mode of Occurrence of some of the Volcanic Rocks of Montana, U.S.A.

Abstract

The volcanic district of the Yellowstone National Park, from which I obtained the various rock-specimens referred to in the foregoing paper, has been already described in detail by Dr. Hayden, Dr. A. C. Peale, and Prof. Bradley, in the reports of the U.S. Geological Survey of the Territories for the years 1871 and 1872. In the autumn of 1878 I had the good fortune to accompany Dr. Hayden and other members of his survey in some parts of the district referred to; and although I have no intention of giving a detailed description of the volcanic phenomena which were observed, a short notice of the localities whence the specimens were obtained, and of the general mode of occurrence of the rocks, may be of some interest as a supplement to Mr. Rutley's description of their microscopic characters. The Yellowstone National Park, within which are the head-waters of two of the great forks of the Missouri (the Yellowstone river and the Madison) and of the Snake river (one of the branches of the Columbia, comprises an area of about 3600 square miles, nearly the whole of which is covered up by volcanic rocks of great thickness.There are very few exposures of underlying sedimentary or other formations; and these are almost entirely limited to the extreme northern edge of the area. Some fifteen miles south of the southern boundary the extension of these volcanic rocks is seen to rest upon the northern spurs of the Teton range of mountains, at which point