270 C. R. KEYES DEVONIAN INTERVAL IN MISSOURI 



ward, until in central Missouri even the main member, the great Cedar 

 Valley limestone, fails altogether. 



The shales in Arkansas preserve only the characters of the upper shale 

 member of the north. There is nothing in this part of the cross-section 

 to represent the limestone members in the more northern sections. 



Around the eastern flank of the Ozarks the Devonian is greatly dimin- 

 ished in thickness, yet it nevertheless retains its characteristic limestone 

 as its major member and is abundantly fossiliferous. 



BASE OF THE CARBONIFEROUS 



The base of the Carboniferous is in this connection assumed to be the 

 bottom of the Chouteau limestone wherever this is present, and when 

 not present, the Burlington limestone or lower member of the Augusta 

 terrane of the Mississippian series. The reasons for considering this 

 horizon at the base of the Chouteau terrane as the plane separating the 

 two great systems of strata are given elsewhere. Whatever may be the 

 real age of the various Interval beds, the horizon here regarded as the base 

 of the Carboniferous is remarkably persistent over a vast area, and, 

 moreover, in sections is everywhere easily recognized. 



Without regard to any possible change in the future in its taxonomic 

 position, the base of the Chouteau must alwa}^s be considered as an im- 

 portant stratigraphic guide-horizon. Throughout the western Interior 

 basin, from the point where it rises out of the ground in northern Iowa 

 and is cut by recent peneplanation of the region south through Missouri 

 and into Arkansas, there is no difficulty is locating this stratigraphic 

 level. It is a feature in the stratigraphy that is conspicuous, ever present, 

 never mistaken. Far to the southwest it appears to retain all its dis- 

 tinctive features unimpaired. 



SUPERIOR LIMIT OF THE SILURIAN 



Throughout the Mississippi valley little hesitancy has been evinced in 

 delimiting the Silurian beds. Whether in the consideration of its litho- 

 logic features or of its faunal characteristics, the separation from over- 

 lying formations is everywhere on sharply drawn lines. This guide- 

 horizon in the general correlation of the strata is easily recognizable even 

 in deep-well sections. Whatever may be the geological age of beds above 

 that may appear obscurely denned or devoid of readily determinable 

 features, one may always feel assured that the deposits will never prove 

 to belong to the Silurian. 



In the upper Mississippi basin, as far south as the Missouri river, these 

 Silurian beds are almost invariably denominated " Niagara dolomites." 

 While, of course, this formation can not be regarded as coextensive with 



