NORTH AMERICAN GEOLOGY. 209 



63. In his second contribution to the study of the Cambrian faunas 

 Walcott discusses the classification of this series in the light of present 

 knowledge of its stratigraphy and fauna. The Cambrian is shown to 

 be as distinctly a system as the Silurian, Devonian, and others, being 

 represented by over 18,000 feet of strata, with ninety- two genera and 

 three hundred and ninety-three species, of which very few pass upward 

 into the Ordovician, or so called Lower Silurian. Its subdivisions into 

 upper, middle, and lower is shown to be required, and although the mid- 

 dle is transitionary in fauna, it presents well-marked characteristics. 

 A detailed description is given of the sections of Georgia, Vermont, 

 and at Eureka, and in the highland ranges, Nevada, where the faunal 

 and stratigraphic distinctions between the Middle or Georgian and the 

 Upper or Potsdam is distinctly exposed. It is also shown that the 

 Upper Cambrian or Potsdam of the Mississippi Valley is fauually dis- 

 tinct from the Middle Cambrian, which, however, is not recognized in 

 this district. The Lower Cambrian is only found east of eastern Mas- 

 sachusetts, and is represented by the faunally characteristic series of St. 

 Johns and Braintree, as there appears to have been a barrier up- to the 

 end of the Lower Cambrian, preventing its extension to the eastward. 

 The following table is given as a classification of North American Cam- 

 brian rocks: 



i Lower portiou of the Calciferous formatiou of 

 Lower Calciferous< New York and Canada; Lower Magnesian 

 ( of Wisconsin, Missouri, etc. 

 r Potsdam of New York, Canada, Wisconsin, 

 Upper Cambrian <( Texas, Wyoming, Montana, and Nevada; 



Potsdam, Knox,! Tonto of Arizona ; Knox Sliales of Tennes- 

 Tonto. I see, Georgia, and Alabama. The Alabama 



section may extend down into the Middle 



^ [ Cambrian. 



Georgia formation of Vermont, Canada, and 



I New York; limestone of L'Anse an Loup, 



Middle Cambrian i Greorgia, L'Anse an ! Labrador ; lower part of Cambrian section 



) Loup Prospect. ] of Eureka and Highland Range, Nevada ; 



I upper portion of Wasatch Cambrian sec- 



l^ tion, Utah. 



fParadoxides beds of Braintree, Mass.; St. John, 



rSt. John, Brain- N. B. ; St. John's area of Newfoundland; 



Lower Cambrian J tree, Newfound- j Lower portion of Wasatch section, Utah. 



1 land, Wasatch, ) The Ocoee conglomerate and slates of East 



L Tennessee (?). | Tennessee are somewhat doubtfully in- 



t eluded. 



The Grand Canon, Chuar, Llano, and Keweenaw series are considered 

 pre-Cambrian on account of unconformity with the unmistakable Cam- 

 brian and their inconspicuous faynas. They are termed the Keweenaw 

 group, and the interval between them and the Upper Cambrian is 

 thought to be a hiatus equal to the Middle and Lower Cambrian ; the 

 Keweenaw not being directly overlain by the two latter as far as known.* 



64. In continuing his investigations in the vicinity of Poughkeepsie, 

 New York, Dwight has discovered an outcrop of fossiliferous Potsdam 



* U. S. Geol. Survey Bulletin (No. 30), vol. 4, pp. 727-1095 ; and Am. Jour. Sci.,iii, 

 vol. 32, pp. 138-157. 



H. Mis. 600 14 



