244 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



epoch the group term will be provisionally used for them. * * * ^ thickness of. 150 feet 

 is present. 



The "Niagara" dolomite is distinguished from that of the Galena not only by the occurrence 

 of certaiu characteristic fossils, of which Halysites catenulatus is probably most widespread and 

 easily recognized, but also by fairly constant lithologic differences. In general, the "Niagara" 

 is much lighter colored, finer textured, and is relatively free from the small in-egular cavities 

 which mark the Galena. 



K-L 12. SOUTHWESTERN MONTANA AND WESTERN WYOMING. 



Silurian strata may be absent in southwestern Montana and western Wyoming. 

 They have not been identified by their fauna. In general the Cambrian*, or the 

 Ordovician where present, is succeeded by Carboniferous, but the Jefferson lime- 

 stone (Devonian) intervenes in some sections and is underlain by strata that appear 

 to represent the Silurian in seemingly conformable sequence. 



Kindle *^^^ studied the distribution of the Jeflferson limestone (see p. 320, Chap- 

 ter VI) and discusses the Silurian in certain sections. For the section on Boulder 

 Creek, west of Princeton, Mont.,, in the Philipsburg quadrangle, he says: 



No fossils were procured from the 200 feet of beds [gray to brownish shale and sandstone, 

 the latter predominant at the base] at the base of the Jefferson Umestone, so that the age of the 

 beds intervening between the Cambrian and Devonian faunas remains undeternnned in this 

 region. No evidence of any stratigraphic break in this part of the section was observed. 



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

 limestone occurs immediately above the following beds: 



Feet. 



E. Shales and covered 30 



D. White limestone 5 



C. Brownish arenaceous thin-bedded magnesian limestone 90 



B. Limestone [Upper or late Middle Cambrian]. 



The beds C, D, and E represent both Ordovician and Silurian, unless there is 

 an undetected hiatus. Kindle says : 



No fossils were obtained from the beds separating this formation [Cambrian] 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 provi- 

 sionally be referred to the Silurian. 



In southwestern Wyoming occurs a similar sequence of Cambrian limestone, 

 unfossiUferous strata, possibly Silurian, and a limestone correlated with the Jeffer- 

 son. Describing a section near Viola, Wyo., Kindle says: 



We are without positive information regarding the age of the 700 feet of gray Hmestones 

 below the saccharoidal Hmestones of the Jefferson. These beds may be in part of the same 

 formation and age as the 40 feet of limestone at their base, the age of which is known by fossils 

 to be Cambrian. It seems probable, however, that Silurian or Ordovician strata, or perhaps 

 both, may be included in the series. The Ordovician is present in the Bighorn Mountains to the 

 north, and both are present in northeastern Utah. 



K-L 17-18. ONTARIO AND WESTERN NEW YORK. 



The belt of Silurian strata which extends from Georgian Bay to western New 

 York comprises the Clinton and other subdivisions of the Niagara, together with 

 the Medina, below the Clinton, and several formations above the Niagara. The 



