264 THE DEVONIAN AND CARBONIFEROUS. [bull. 80. 



alents in several of the States of the interior a black shale is conspicu- 

 ously constant. While the black shale was generally correlated as 

 Devonian, its precise age has not up to the present time been certainly 

 fixed. 



That the black shale has not been satisfactorily correlated is shown 

 by its retention of that general name in spite of its frequent correlation 

 with other black shales of definite age, as the Marcellus and the Gen- 

 esee formations of New York. 



As the terrane separating Silurian from Carboniferous thins out to 

 the southwest, it is finally restricted to a few feet of black shale, but it is 

 not proved paleontologically precisely what part of the expanded series, 

 called Devonian in New York and Ohio, is represented by this shale. 



In the later work of the geologists of Ohio a certain symmetry in 

 correlation is sought by uniting the black shales, up to and including 

 the Cleveland shale, into a single group and calling it the Ohio shale, 

 correlating this as the upper member of the Devonian system. 1 



Prof. Newberry, in his monograph on " The Paleozoic fishes of North 

 America," 2 classifies the deposits above the last prominent black 

 shale as Carboniferous, thus conforming with the general principle 

 of making the black shale the top member of the Devonian system. 

 In the case of Prof. Newberry this correlation is not new, and was 

 first advanced to make the classification conform to a theoretical order 

 of deposits explained under the name " circles of deposition." 3 But 

 the tendency on all hands has been to accept this structural line of de- 

 markation between the Carboniferous and Devonian formations. Still 

 further work upon the structural as well as the paleontologic features 

 of these black shales will be needed to determine their true correlation. 



The subdivision of the Mississippian series is a matter of classifica- 

 tion rather than correlation proper. All through the province varia- 

 tions in the stratigraphy are seen in the development of the local 

 geologic structure. The structural or lithologic formations distinguish- 

 able over most of the province are as follows : 



Chester group. Worthen. Burlington limestone, Hall. 



St. Louis group, Worthen. Kinderhook group, Meek and 



Warsaw limestone, HalL Worthen ; or 



Keokuk group, Worthen. Chouteau group, Broadhead. 



These formations have been defined in their typical localities and the 

 faunas as locally studied have been described, but in several cases dif- 

 ficulty has been experienced in attempting to extend the classification 

 over the whole Mississippian province. 



The difficulties have occurred most frequently in distinguishing be- 

 tween Burlington and Keokuk faunas in the formations in western and 



1 Geol. Survey of Ohio, vol. 6, by Edw. Orton, 1888. 

 • 2 U. S. Geol. Survey, Monograph, vol. 16, 1889. 



3 See a theory of circles of sedimentation, by J. S. Newberry, Am. Ass. Adv. Sci., Proc, vol. 22, pt. 2, 

 pp. 185-196, 1873, and on circles of deposition in sedimentary strata, Canadian Nat., new series, vol. 7, 

 pp. 163-164. 



