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

Cover

Introduction

Geological sketch of the region

Electric Peak

Geological description
     Geological map
The eruptive rocks of Electric Peak
     Use of the terms porphyrite and porphyry
     Sheet rocks
     Dike and stock rocks
          The dike rocks and certain contact facies of the stock
          The stock rocks and apophyses
          Intergrowth of hornblende and pyroxene in glassy rocks
          Quartz-mica-diorite-porphyrite
     General consideration of the mineral and chemical composition of the rocks

Sepulchre Mountain

Geological description
The volcanic rocks of Sepulchre Mountain
     The lower breccia
     The upper breccia
     The dike rocks
     General consideration of the mineral and chemical composition of the rocks

Comparison of the rocks from the two localities

Correlation of the rocks on a chemical basis
Effect of mineralizing agents
Application to the classification of igneous rocks

Appendix


ILLUSTRATIONS

PLATE

XLVI. Electric Peak from Sepulchre Mountain
XLVII. Head of East Gulch of Electric Peak
XLVIII. Fig. 1. Diorite (coarse grain)
             Fig. 2. Diorite (medium grain)
XLIX. Fig. 1. Granite (fine grain)
             Fig. 2. Quartz-mica-diorite-porphyrite
L. Intergrowths of minerals in the diorite
LI. Intergrowths of minerals in glassy rocks and quartz phenocrysts
LII. Sepulchre Mountain from its northwest spur
LIII. Geological map of the region

FIGURE

79. Variation in silica percentages
80. Diagram showing molecular variation of the rocks at Electric Peak
81. Diagram showing molecular variation of rocks at Sepulchre Mountain

TABLES

TABLE

I. Mineral variation of the porphyrites at Electric Peak

II. Mineral variation of the diorites and their facies at Electric Peak

III. Mineral variation of the dike rocks at Electric Peak

IV. Grades of crystallization of the dike rocks at Electric Peak

V. Mineral variation of rocks of subgroup IIa

VI. Mineral variation of rocks of subgroup IIb

VII. Mineral variation of rocks of subgroup IIc

VIII. Grades of crystallization of the stock rocks

IX. Chemical analyses of the rocks of Electric Peak

X. Silica percentages of the rocks of Electric Peak

XI. Molecular variation of the essential minerals of the diorite

XII. Mineral variation of the upper breccias of Sepulchre Mountain

XIII. Mineral variation of the dike rocks of Sepulchre Mountain

XIV. Grades of crystallization of the rocks of Sepulchre Mountain

XV. Chemical analyses of the rocks of Sepulchre Mountain

XVI. Order of eruption of the rocks at Electric Peak and Sepulchre Mountain

XVII. Correlation of the rocks on a chemical basis

XVIII. Grades of crystallization of all of the rocks

XIX. Original specimen numbers corresponding to those used in this paper


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Last Updated: 23-May-2015