CD. Walcott — Position of the Olenellus Fauna. 383 



SILURIAN (ORDOVICIAN). 





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Lower Calciferous. 



Lower portion of the Calciferous sandrock of 

 New York and Canada; Lower Magnesian 

 limestone of "Wisconsin, Missouri, etc. 





Potsdam 



Knox 



Tonto 



Potsdam sandstone of New York, Canada, Wis- 

 consin, Texas, Wyoming; Gallatin limestone 

 of Montana and portion of Pogonip limestone 

 of Nevada; Knox shales of Tennessee ; Coosa 

 shales of Georgia and Alabama; the Alabama 

 section may extend down into the Middle Cam- 

 brian. Tonto calcareous shales of Arizona. 



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Bell Isle 



Shales and sandstones of Great and Little Bell 

 and Kelley's Islands, Conception Bay, New- 

 foundland. 







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St. John 



Shales and slates of Braintree, Massachusetts; 





Braintree 



Avalon _ 



St. John, New Brunswick and the Avalon 

 Peninsula of Newfoundland. Central portions 

 of the New York and Nevada Cambrian sec- 

 tions. 





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Georgia 



Georgia shales and "Granular Quartz " of Ver- 





Terra Nova 



Prospect, 



mont, Canada, New York and Massachusetts. 

 Limestones, etc., of L'Anse au Loup, Labrador; 

 northwest coast and Peninsula of Avalon, 

 Newfoundland; lower part of Cambrian sec- 

 tion of Eureka and Highland Range, Nevada ; 

 Upper arenaceous shales of Big Cottonwood 

 Canon, Cambrian section of Utah. 



ALGONKIAN. 



The topmost division, Lower Calciferous, includes the passage 

 beds between the Cambrian and subjacent Lower Silurian (or 

 Ordovician) systems. In northeastern N'ew York the line 

 between the two systems is very distinct, but in central New 

 York and ISTevada there is no stratigraphic break between them. 

 Passage beds must necessarily exist in some localities ; in the 

 table these are recognized by the Lower Calciferous, as in 

 Wisconsin that horizon contains a fauna more intimately 

 related to the Cambrian than to the superjacent second fauna. 



The names Potsdam, A cadian and Georgian might be used 

 in the second column to replace Upper, Lower and Middle 

 Cambrian. The objection, for instance, that the typical Aca- 

 dian fauna is not present in the great Rocky mountain area, 

 and that there is a Middle Cambrian zone which is recognized 

 there, leads me to drop the denotive names in the second 

 column and use the more universally applicable terms Upper, 

 Middle and Lower Cambrian. 



