PROGRESSIVE OVERLAP 599 



ular sandrock, and this by the Choteau limestone. In the southwestern 

 section the Black shale is also succeeded by a compact limestone (the 

 Sac), followed by a vermicular sandstone (the North View, identical in 

 character with the Hannibal, and formerly identified with it by the 

 Missouri geologists) ; and finally by a fine grained buff limestone (Pierson 

 limestone) representing the Choteau of the northeastern section. In 

 actual age the Sac, North View, and Pierson of the southern section are 

 equivalent to the Hannibal and Choteau of the northern one. This 

 similarity of lithic succession strengthens the case and makes it prac- 

 tically certain that we have in the Black shale of the Mississippi valley 

 a basal bed of a transgressing sea, and that the age of this basal bed, as 

 in the case of basal sandstones, varies from place to place, rising south- 

 ward in the series. Compare figure 4.* 



"With this demonstrated example before us, we may next consider the 

 Black shale of the southern Appalachians, which is universally regarded 

 as of Devonic age in all of its exposures. Ulrich has recently restated 

 his convictions in this matter by proposing to apply the name Ohio shale 

 to this formation wherever found. Even if this shale represented only 

 Devonic beds in its different outcrops, the fact remains, and is recognized 

 by Ulrich, that it does not represent the same portion of the Ohio series 

 in all its exposures. That part generally known as the Chattanooga 

 shale is regarded by Ulrich as representing the upper part of the shale 

 in Ohio, and to call this small portion by the name of the whole is at 

 least a questionable proceeding. 



The Devonic age of the Chattanooga shale may, however, be seriously 

 doubted. The establishment of this age is not based upon fossils; for 

 those found — a few Lingulas, doubtfully referred to L. spatulata of the 

 Genesee, and some Conodonts — are wholly inconclusive. Ulrich himself 

 says: 



"Although there is little besides stratigraphic position and lithologic char- 

 acters on which to base the reference of this black shale to the Ohio formation, 

 it is so referred with the utmost confidence. In every feature this [Hardin 

 County] shale is practically identical with many of nearly a hundred exposures 

 of this formation examined by the writer in Kentucky, Tennessee, and Ohio. 

 From lake Erie southward to northern Georgia and westward to this district 

 the Ohio shale is remarkably constant in its lithologic characters. Despite this 

 constancy this formation has received a number of names. The name Ohio 

 shale, proposed by Andrews, the oldest of the geographic names applied to 

 this formation, is here adopted."! 



* Compare this similarity of succession with that found in the chalk of England and 

 Ireland, cited above. K, 



t Professional paper no. 36, p. 25. 



LV — Bull. Geol. Soc. Am., Vol. 17, 1905 



