CAMBRIAN FOSSILS FROM THE PIOCHE MOUNTAINS, 



NEVADA 



FRED J. PACK 



THE FOSSILIFEROUS HORIZONS 

 The fossils which are used as a basis for this contribution were 

 collected by the writer in the fall of 1905 and the early part of 1906. 

 Mr. Charles D. Walcott visited the district in 1885, and discovered a 

 very fossiliferous horizon which he has correlated with the Middle 

 Cambric. He also gathered a supposed Lower Cambric fauna from 

 a point about six miles east of Pioche. From both these places he 

 obtained several forms that had not been previously described. He 

 has recorded the results of his investigations in Bulletin No. jo of the 

 United States Geological Survey. So far as the writer is aware, no 

 other study has been made of this fauna. 



Some interesting problems have arisen in the attempt to correlate 

 the Pioche section with the horizons elsewhere. In America the 

 Cambric of the eastern states has received by far the most attention ; 

 it has been divided into zones, each of which is represented by a char- 

 acteristic fauna. The following subdivisions are generally recognized: 



Upper Cambric— Dikellocephalus zone 

 Middle Cambric— Paradoxides zone 

 Lower Cambric — Olenellus zone 



In Nevada the upper and the lower zones are represented by forms 

 which are almost identical with those of similar horizons in the East, 

 but the middle one is characterized by an almost entirely new series. 

 Olenellus and Dikellocephalus are separated in every case by more 

 than 1,000 feet of conformable strata, which carry in places the new 

 forms and some old ones, none of which are elsewhere typically rep- 

 resentative of any definite horizon. This condition has made- corre- 

 lation difficult; Walcott, however, has referred these formations to 

 the Middle Cambric. Dr. G. F. Matthew does not concur in this 

 opinion, but thinks that they properly belong to the Upper Cambric. 



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