﻿89 Outcrops 



Hypothyris globularis n. sp. 



Atrypa missouriensis 



A. reticularis 



Athyris parvula 



A. montanensis n. sp. 



Spirijer Occident alls n. sp. 



5. engelmanni 



S. argentarius 



S. utahensis 



Loxcmema approximatum? 



Straparollus sp. 



About 20 miles south of Princeton, on the east fork of Rock 

 Creek, nearly the entire thickness of the Jefferson limestone is 

 exposed. The lower 300 feet of the Madison limestone in this 

 section is more or less shaly. The contact of the Jefferson lime- 

 stone and the overlying formation is not exposed in the Rock 

 Creek section. The beds immediately below the dark limestones 

 of the Jefferson formation as seen in the Rock Creek gorge are 

 given in the section following: 



Section on East Fork of Rock Creek, Montana 



Feet 



E. Shales and covered 30 



D. White limestone 5 



C. Brownish arenaceous thin-bedded magnesian limestone 90 



B. Limestone banded with thin wavy laminae of brown to gray 

 shale, the latter usually in .K-inch bands interbedded with 



gray limestone in 2-inch to 3-inch strata 250 



A. White magnesian limestone 225 



The peculiar laminated limestone (B of the section) contains a 

 fauna in which trilobite fragments are abundant. This lime- 

 stone has the same lithologic and faunal characteristics as 

 division B of the Boulder Creek section. The fauna of these beds 

 is identical with that occurring in B of the Boulder Creek section 

 and represents basal Upper Cambrian or late Middle Cambrian, 

 according to Mr. E. O. Ulrich, who examined the collection made 

 by the writer. 



