﻿i2 Bulletin 20 90 



No fossils were obtained from the beds separating this forma- 

 tion from the Jefferson in either the Rock Creek or Boulder 

 Creek sections. These beds, measuring 210 feet in thickness in 

 the Boulder Creek section and 125 feet in the Rock Creek section 

 may provisionally be referred to the Silurian. 



From the lower 50 feet of the Jefferson limestone in the Rock 

 Creek section the following fossils were found : 



Schuchertella chemungensis var. arctostriata 



Stropheodontaci. macrostriata 



Spirifer engelmanni 



Other sections in southwestern Montana might be given which 

 would present the same general sequence with reference to the 

 Jefferson limestone. All the sections show a limestone series in 

 which dark to nearly black limestones predominate, followed by 

 the Madison limestone, which is locally separated from the 

 Jefferson by the Threeforks formation. The Jefferson in all the 

 Montana sections is preceded by limestones and shales in which 

 the latest observed faunas are of Cambrian age. 



Sections in Wyoming. — The Montana sections show a close 

 correspondence in these characteristic features to the south- 

 western Wyoming section, which follows. This section occurs 

 in the westward-dipping beds of Labarge Mountain, northeast of 

 Viola post office and west of the Sayles coal mine. 



Section Northeast of Viola, Wyoming 



J. Light to dark-gray limestone, oolitic in lower 20 feet (Madi- 



sonlimestone) 500 =*= 



I. Drab shales and shaly, thin-bedded magnesian and siliceous 



limestone 80 



H. Buff to gray, limestone with much black magnesian limestone 

 in the upper part, saccharoidal in texture, and weather- 

 ing with roughly pitted surface; covered in part (Jeffer- 

 son limestone) .......' ioog =*= 



G. Gray limestone, partly covered 700 =*= 



F. Gray limestone, with Cambrian trilobites abundant 40 



E. Green shale and covered 300 =■= 



D. Drab shale and covered 30 



C. Thin-bedded gray limestone, with Cambrian trilobites 



