walcott.1 CORRELATION. 413 



Black Hills of Dakota." where a sandstone characterized by Upper 

 Cambrian fossils is identified without comment as the Potsdam sand- 

 stone. 



U. S. Geological Survey. — In the consolidation of the various Gov- 

 ernment surveys and their reorganization under one head in 1879 the 

 opportunity for broad comparisons and studies was increased, and the 

 opportunities were still further increased in 1881, when the survey was 

 extended to include the entire area of the United States. In the Rocky 

 Mountain Province the correlations of the fortieth parallel survey 

 were accepted, and extended in the abstract of the report on the geol- 

 ogy of the Eureka district of Nevada. 1 In the report on the paleontology 

 of the district the paleontologic data upon which the correlations were 

 made are assembled and described. 2 In this it is shown that while corre- 

 lations between the great groups may be extended between the Appa- 

 lachian Province and the Rocky Mountain area, it is impossible to 

 correlate the minor divisions of the groups. 



In 1886 an extended correlation of the rocks and fossils from the 

 Lower (Jambrian, then called Middle Cambrian, was published in Bul- 

 letin No*. 30 of the U. S. Geological Survey. 3 Typical sections are 

 given of the Cambrian rocks of the Atlantic Coast, Appalachian, and 

 Rocky Mountain Provinces, with the names of the species of fossils 

 occurring in the various zones, and on page 44 the various sections are 

 correlated and grouped in a table to show their relative range. The 

 principles of correlation are not enunciated, but those used, as shown 

 by the text, are the strati graphic position of the faunas in relation to 

 each other in the same province and a comparison of the faunas in one 

 area with those of another. 



In a more recent work 4 the correlations are made between the various 

 horizons of the Cambrian of the United States on the stratigraphic 

 position of the strata and the occurrence of similar groupings of organic 

 remains. It is observed that the presence of the Olenellus fauna is 

 regarded as the strongest factor in the correlation of the Lower Cam- 

 brian, and it is assumed that the presence of this fauna indicates the 

 same geologic horizon. It is not implied that the formations charac- 

 terized by the Olenellus fauna are strictly contemporaneous wherever 

 it has been found. On the contrary, its presence in such widely sepa- 

 rated localities as England, France, Spain, Nevada, and British Colum- 

 bia is considered as prima facie evidence that the deposits are not 

 strictly contemporaneous. That they were contemporaneous in a geo- 

 logic senseor that they occupy the same relative position in the geologic 

 series is accepted throughout the paper. 



1 Third Ann. Rept. TJ. S. Geol. Survey, 1883, pp. 248-272. 



1 MoDg. U. S. Geol. Snrvoy, vol. 8, 1884. 



8 Walcott, C. D. : Second contribution to the studies of the Cambrian faunas of North America. 

 1886. 



4 Walcott, C. D. : The fauna of the Lower Cambrian or Olenellus zone. Tenth Ann. Rept. V. S. Geol, 

 Survey, 1800, pp. 509-763. 



