BULL. 6E0L. SOC. AM 



VOL. 21, 1909, PL. 24 



FiGDUK 1. — A Natural Bridge in Miocene Shales, Santa Cruz County, California 

 Formed by wave erosion. Photograph by Arnold, U. S. Geological Survey 



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1 



Figure 2. — A Natural Bridge op Yellow Sandstone spanning a small Brook on 

 Lookout Mountain^ Tennessee 



Formed by the widening of a joint 



ILLUSTRATIONS OF NATURAL ROCK BRIDGES 



