THE ORDOVICIAN PERIOD. 



315 



mined. As implied above, it is improbable that there is any exact 

 equivalency between the eastern and the interior series. 1 



The classifications of New York and Wisconsin have been adapted, 



-^_;r J ^Sm 





Fig. 134. — Trenton limestone near Fort Snelling, Minn. Shows two systems of 



joints. (Calvin.) 



with but little change, to the Ordovician formations of most of the 

 area between the Appalachian mountains and the Great plains. 



The Appalachian sections. 2 — In other parts of the country, the 

 subdivisions of the system are very different. In the Appalachian moun- 

 tains of Tennessee (see section in Appendix, vol. Ill), a series of lime- 



1 In Wisconsin, there is an unconformity between the Lower Magnesian limestone 

 and the St. Peters sandstone. This and other considerations raise the question 

 whether the Lower Magnesian formation should not be classed with the Cambrian, 

 rather than with the Ordovician. 



2 For local details in the Appalachians, see the following folios U. S. Geol. Surv. : 

 Gadsden, Ala.; Stevenson, Ala.-Ga.-Tenn. ; Ringgold and Rome, Ga.; Estillville, 

 Ky.-Va.-Tenn. ; Briceville, Chattanooga, Cleveland, Columbia, Kingston, Knoxville, 

 Loudon, Maynardville, and Morristown, Tenn. ; Bristol, Va. ; Harper's Ferry, Va.- 

 Md.-W. Va. ; Monterey, Pocahontas, Staunton, and Tazewell, Va.-W. Va. ; Franklin 

 and Piedmont, W. Va.-Va. In most of these folios, the Ordovician is classed 

 with the Silurian under the latter name. The text of the folios frequently distin- 

 guishes between the Lower Silurian (Ordovician) and the Upper Silurian (Silurian). 

 See also the geological reports of Alabama (especially Hayes's Rept. 1892), Georgia, 

 Tennessee, Kentucky, and Pennsylvania (especially Summary of Final Rept., Vol. I). 



