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vals in the nonmagnesian series, whereas the Halysites is found in 

 the most highly magnesian beds of the series. It was clearly 

 contemporaneous with the Devonian fauna. In the Logan 

 section H . catenulatus occurs above beds holding the following 

 species : 



Favosites cf . limitaris 



Martinia maia 



Pleurotomaria sp. 



Naticopsis sp. 



Tropidocyclas sp. 



Spirorbis sp. 



The presence of M. maia, which is by far the most abundant 

 species in this faunule, indicates clearly that a Devonian horizon 

 is represented. 



Correlation 



Previous correlations. — Owing to the extreme scarcity of fossils 

 which generally characterizes the Jefferson limestone, all the 

 references to the age of the formation which occur in the litera- 

 ture are either largely provisional or purely hypothetical. In 

 five of the papers in which the formation is discussed it is re- 

 ferred to the Silurian 13 '; in three others it is called Devonian 14 ; 

 and in two others, including the most recent reference 15 , these 

 beds are placed in a Devono-Silurian series 16 . 



Only two of these papers contain paleontologic data regarding 

 the age of the beds. Peale 17 obtained Devonian fossils about 30 



I3 Weed, Geologic Atlas U. S., folio 30, U. S. Geol. Survey, 1896, 

 Weed and Pirsson, Bull. U. S. Geol. Survey No. 139, 1896, p. 37. Weed, 

 eighteenth Ann. Rept. U. S. Geol. Survey, pt. 3, 1898, pp. 469-470. 

 Hague, Geologic Atlas U. S., folio 52, U.S. Geol. Survey, 1899, p. 2. Iddings 

 and Weed, Mon. U. S. Geol. Survey, vol. 32, pt. 2, 1899, pp. 1-164. 



,4 Peale, Bull. U. S. Geol. Survey No. no, 1893, p. 27. Weed, 

 Geologic Atlas U. S., folio 1, U. S. Geol. Survey, 1894, p. 2. Peale, 

 Geologic Atlas U. S., folio 24, U. S. Geol. Survey, 1896. 



'SEmmons, Bull. U. S. Geol. Survey No. 315, 1907, p. 34. 



l6 Weed, twentieth Ann. Rept. U. S. Geol. Survey, pt. 3, 1900, p. 287. 



I7 Bull. U. S. Geol. Survey No. no, 1893, p. 29. 



