GEOLOGY OF THE SUN RIVER DISTRICT, MONTANA 559 



On the eastern ridge is a softer, brownish limestone containing: 



A try pa missouriensis Miller, C. 

 Spirijer coniculus Girty, C. 



The Spirifer indicates an Ouray (Devonian) age. 



The Atrypa has been reported from the Three Forks (Devonian) 

 shale in which it is abundant in the Yellowstone National Park 

 area. It occurs in a finer-grained and slightly browner limestone 

 than does the Spirifer. These two fossils have not been noted on 

 the same piece of rock. 



Locality 5. — The easternmost reef consists, according to Pro- 

 fessor Crosby, of limestone containing the same corals — Zaphrentis, 

 Syringopora, etc. — -as are present at locality 3 and at the western 

 ridge of Arsenic Reef, locality 4. 



This limestone reef is, according to Professor Crosby, overthrust 

 upon the Cretaceous beds to the east. One-fourth of a mile from 

 the eastern edge of the reef a vertical drill core showed the Creta- 

 ceous shale beneath the limestone. The Cretaceous near the 

 limestone is slightly folded, but farther east it is approximately 

 horizontal. 



