NORTH AMERICAN GEOLOGY. 209 



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

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

 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 aud 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 detailod description is given of the sections of Georgia, Vermont, 

 and at Eureka, and in the highland ranges, IS'evada, 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 faunally 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 Il^orth American Cam- 

 brian rocks: 



( i Lower portion of the Calciferous formation of 



Lower Calciferous-^ New York and Canada; Lower Magnesian 

 of Wisconsin, Missouri, etc. 



r Potsdam of New York, Canada, Wisconsin, 

 Texas, Wyoming, Montana, and 



Upper Cambrian.^ Texas, Wyoming, Montana, and Nevada; 



Potsdam, Knox,! Tonto of Arizona ; Knox Shales ofTenues- 



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



I section may extend down into the Middle 



t Cambrian. 



r Georgia formation of Vermont, Canada, aud 



New York; limestone of L'Ause au Loup, 



•M-^/j/JiflnoTniM-ior, 5 Georgia, L'Anse au J Labrador ; lower part of Cambrian section 



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



I u^jper 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 ^^^^^ Newfound- J Lower portion of Wasatch section, Utah. 



j land, Wasatch, i The Ocoee conglomerate and slates of East 



(^ Tennessee (?). j Tennessee are somewhat doubtfiilly in- 



1^ 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 faunas. They are termed the Keweenaw 

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

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

 Keweenaw not being directly overlain b^^ 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., m, 

 vol. 32, pp. 138-157. 



H. Mis. 600 14 



