﻿99 Correlation 21 



limestone in Nevada. Seven other species, or about one-third of 

 the known fauna of the Jefferson limestone, occur also in the 

 fauna of the Nevada limestone. So large a percentage of 

 species common to the faunas of these two limestones leads to 

 the conclusion that the two faunas are identical and that the 

 Jefferson limestone of Montana and Utah is the stratigraphic 

 equivalent of the Nevada limestone of Nevada. The only pos- 

 sible grounds for a different conclusion would be the presence of a 

 fauna above or below the Nevada limestone having an equal or 

 greater resemblance to the Jefferson limestone fauna. Walcott's 20 

 work on the Nevada faunas clearly indicates that there is no such 

 fauna. The White Pine shale which follows the Nevada lime- 

 stone is now known to be of Carboniferous age and contains, as 

 indicated by Walcott 21 , but one species which is common to the 

 Nevada limestone — Productus subactdeatus . The Nevada lime- 

 stone fauna is preceded in the section by a fauna of Silurian or* 

 Ordovician age 22 . 



The fauna of the Threeforks shale, which immediately follows 

 the Jefferson limestone in some of the sections in Montana, is 

 composed for the most part of alien species. Dr. Raymond's 23 

 list of the Threeforks shale fauna at Logan and Threeforks, 

 Montana, shows but three species which are known in the Jeffer- 

 son limestone. Evidently very few of the indigenous species of 

 the Jefferson limestone survived the conditions which inaugura- 

 ted the deposition of the shales that terminated the Devonian in 

 this region. 



Dr. Girty 24 has described a small Devonian fauna from the 

 Threeforks limestone of the Yellowstone Park. As pointed out in 

 a foot note p. 4, Threeforks limestone and Threeforks shale have 



^Paleontology of the Eureka district. Mon. U. S. Geol. Survey, vol. 

 8, 1884, pp. 283, 284. 



2I Ibid, p. 5. 



"Ibid, p. 4. 



23 0n the occurrence in the Rocky Mountains of an Upper Devonian 

 fauna with Clymenia. Am. Jour. Sci., vol. 23, 1907, pp. 120, 121. 



24 Devonian and Carboniferous fossils from the Yellowstone National 

 Park. Mon. U. S. Geol. Survey, vol. 32, pt. 2, 1899, pp. 479-507. 



