DEVONIAN. 323 



dark magnesian limestones are the predominant element, although nonmagnesian limestones 

 and in some places argillaceous rocks appear as minor components of the formation. In the 

 Carboniferous the dark magnesian limestones of the lower series are replaced by lighter-colored 

 nonmagnesian limestones. The Carboniferous and Devonian series of these sections show about 

 the same kind and degree of contrast which is found between these series in the Montana sec- 

 tion. The Devonian beds of these northern Utah sections appear 'without question to be the 

 southern continuation of the Jefferson limestone. The following is a representative section 

 occurring nearly east of Paradise post office, in Green Canyon: 



Section in Green Canyon, Utah. 



Feet. 



D. Gray nonmagnesian limestone, partly covered 900± 



0. Dark-gray to black magnesian limestone, generally with saccharoidal texture l,100zt 



B. Thin-bedded limestone, buff or brownish near top, with peculiar concretionary develop- 

 ment, with thin-bedded bluish-gray limestone in lower part • 100 



A. White to light-gray magnesian limestone, with chert or siliceous beds locally developed 



[Silurian, Kindle] 150 



2,250 



Kindle mentions the contrast between the Jefferson fauna and that of the Ouray 

 limestone of Colorado, and also comments on the close equivalency of the Jefferson 

 limestone of Montana, Wyoming, etc., and the Nevada limestone of Nevada and their 

 faunas. He says in part: 



The fauna of the Jefferson limestone as known at present is a small one, numbering about 

 32 species. Like many other western faunas, it includes very few species which are common to 

 the standard sections east of the Mississippi. Of the five species which are common to the weU- 

 known eastern sections all except Atrypa reticularis are characteristic Devonian species, a 

 sufficient number to make evident the Devonian age of the fauna. * * * 



In attempting to determine just what part of the Devonian is represented by the fauna 

 of the Jefferson limestone we find that two of the five species which are common to the Jefferson 

 limestone fauna and the eastern Devonian fauna are Atrypas. One of these, A. reticularis, has 

 no diagnostic value, while A. spinosa has a recorded range from the Corniferous to the Chemung. 

 The other three species, ScJiucTiertella chemungensis arctostriatus, Productella spinulicosta and 

 Martinia ms,ja, are known in the eastern sections only in Middle Devonian horizons. The 

 absence from the fauna of any of the large Productellas, which generally characterize the late 

 Devonian faunas, together with the presence of an Upper Devonian fauna following it in the 

 section, both supplement the intrinsic evidence of the fauna that it is earlier than Upper Devo- 

 nian. The known range of the fossils which are common to eastern sections suggests a Middle 

 Devonian age for at least a part of the Jefferson limestone. The fauna cojitains no coarsely 

 plicated Spirifers or other fossils generally characteristic of the early Devonian but other 

 evidence strongly supports the view that the Jefferson limestone represents both Lower and 

 Middle Devonian. 



No evidence of a stratigraphic break between the Jefferson and the preceding formation 

 has been observed by the writer in any of the sections examined by him. None of the fohos 

 which describe the Jefferson limestone record any unconformity at its base. * * * j^^ 

 Utah the writer has found conditions that seem to indicate there the continuity of sedimentation 

 from the Silurian to the Devonian, which, although probable in Montana, has not been demon- 

 strated. * * * 



The fauna of the Threeforks shale, which immediately follows the Jefferson Hmestone in 

 some of the sections in Montana, is composed for the most part of alien species. Dr. Eaymond's 

 list of the Threeforks shale fauna at Logan and Threeforks, Mont., showed but three species 

 which are known in the Jefferson limestone. Evidently very few of the indigenous species of 

 the Jefferson limestone survived the conditions which inaugurated the deposition of the shales 

 that terminated the Devonian in this region. 



