BULL. GEOL. SOC. AM. 



VOL. 21, 1909, PL. 23 



Figure 1. — A petrified Log forming a Bridge across a Ravixe in the petrified 

 Forest of Arizona near Adamana 



Figure 2. — Natural Bridge^ Big Bad Lands^, South Dakota 

 Photograph hy Darton, U. S. Geological Survey 



ILLUSTRATIONS OF NATURAL ROCK BRIDGES 



