Geological Survey 12th Annual Report (Part I) The Eruptive Rocks of Electric Peak and Sepulchre Mountain, Yellowstone National Park

For the student of igneous or eruptive rocks there is no question which excites greater interest, and the correct answer to which would be of greater importance, than that involving the relation or connection between the various forms and kinds of coarse grained rocks and the different varieties of glassy ones. Any group of observations, therefore that bears upon this problem should be studied with the greatest care, in order that we may learn how far it contributes to our understanding of these intricate relations, which not only lie at the foundation of any system of classification of igneous rocks, but which must affect our comprehension of the real nature of the rocks themselves.

The observations recorded in the following pages appear to contribute so largely to certain phases of the problem that it is hoped they may be presented in such a manner that the reader will be able to judge whether the conclusions arrived at by the writer are sufficiently well founded.

This study forms a part of the work undertaken by the division of the U. S. Geological Survey under the charge of Mr. Arnold Hague, which has been investigating the region of the Yellowstone National Park. It constitutes a chapter of the contributions which are being made from time to time to the knowledge of this highly attractive region, where the character of the country is so diversified that the student is confronted by nearly every phase of geology, among the most prominent of which are the phenomena of volcanic action, including the distribution and character of the volcanic material, the physics and chemistry of the thermal springs, and the dynamics of erosion and glaciation—problems which are being investigated by different members of the division. The present paper deals with a group of eruptive rocks occurring at Electric Peak and Sepulchre Mountain, which has been studied with special care because of the bearing of the results upon the general petrological question already stated.

The eruptive rocks of Electric Peak and vicinity embrace a group of intrusive rocks that occur in the form of a stock with apophyses, breaking up through Cretaceous strata, which had already been penetrated by horizontal sheets of intrusive rocks from a neighboring center of eruption. They also include a group of extrusive or volcanic rocks, lying east of Electric Peak, which form the mass of Sepulchre Mountain and contain certain intruded bodies.

The object of the present paper is to describe the nature and occurrence of these intrusive and extrusive bodies and to trace the geological and lithological connections between them, and to show the development of crystallization and the resulting mineral constitution of magmas of similar chemical composition which have solidified under a variety of physical conditions.