i8 



SMITHSONIAN MISCELLANEOUS COLLECTIONS \"OL. 68 



Fic. ift. — Looking down into Tokumm Creek caii.\uii 1il-1mu im- i,iil> 

 shown in fig. 12. As the result of the wearing of very deep and large 

 potholes in the massive Cambrian limestones the extended lower portions 

 of adjoining potholes have been united, leaving a natural bridge near their 

 upper margin far above the rushing torrent below. Photograph by Wal- 

 cott, 191 7. 



