BULL. GEOL. SOC. AM. 



VOL. 21, 1909, PL. 21 



Figure 1. — Bridge of Lithoidal Rhyolite, Bridge Creek, Yellowstone Park 

 Photograph Iby Walcott, U. S. Geological Survey 



Figure 2. — Vertical, Platy Structure of the Rhyolite 



This structure made it possible for the stream, with the assistance of frost, to make 

 the Yellowstone National Park bridge. Photograph by Iddings, IT. S. Geological Survey. 



ILLUSTRATIONS OF NATURAL ROCK BRIDGES 



