Page Frontispiece Midway Geyser Basin II Plate 1. Geologic map of Yellowstone National Park 36 Figure 1. Geographic map of Yellowstone National Park 2 2. Index map showing photograph localities 3 3. Skyline of the Gallatin Range in northwestern Yellowstone National Park 6 4. View north along the Yellowstone River and Hayden Valley toward the Washburn Range 6 5. The rocks of Yellowstone National Park 8 6. The geologic time scale 10 7. View downstream along the Lamar River and closeup view of Precambrian gneiss 12 8. Positions of seaways and landmasses during the middle part of Permian time 14 9. Crowfoot Ridge in the southern Gallatin Range 15 10. Mount Everts 16 11. The faunal succession in sedimentary rocks 17 12. Beds of limestone along Pebble Creek and closeup views of outcrop and fossils 18 13. Common kinds of geologic structures produced by deformation of the earth’s crust 20 14. Geologic structures in Yellowstone National Park 22 15. Intrusive and extrusive igneous rock bodies 24 16. The Absaroka volcanoes and their rocks 25 17. Massive beds of volcanic breccia of the Absaroka volcanic rocks and closeup view of outcrop 26 18. Massive layered breccias of the Absaroka volcanic rocks at Barronette Peak 28 19. Giant petrified tree trunks in Yellowstone’s fossil forest 29 20. Closeup view of a specimen of intrusive igneous rock 30 21. Bunsen Peak, a body of intrusive igneous rock 31 22. Outline of the Yellowstone caldera 35 23. Various stages in the development of the Yellowstone caldera 40 24. Extent of the rhyolite welded tuffs that once covered Yellowstone National Park 42 25. The Yellowstone Tuff at Golden Gate and closeup views of tuff specimens 43 26. Cross section through the Mount Washburn-Canyon area, showing relationships along north edge of the Yellowstone caldera 44 27. View southeast across Yellowstone Lake toward the Absaroka