EVIDENCES OP UNCONFORMITIES 29 L 



dome, along the Mississippi river, both Silurian and Devonian sequences 

 are unbroken. 



It has thus been assumed that the Devonian beds form one of the con- 

 centric zones around the older central rocks of the Ozark dome. On 

 this account chiefly it has been urged that the Devonian sediments were 

 laid down around the margins of what is titled the Ozark isle. The 

 necessary inference has been that during Devonian times subaerial 

 erosion took place over the Ozark region. 



Attention to a few facts quickly shows the fallacy of such a hypothesis. 

 The Devonian sediments themselves display nowhere a coarse littoral 

 character. The " concentric ring " is not an unbroken one; it is sun- 

 dered on the southeast and in the northwest portion. Devonian de- 

 posits, highly fossiliferous, occur on the highest part of the Ozark uplift, 

 proving beyond all doubt that in the absence of other evidence they 

 extended undisturbed over the entire dome. It is clearly manifest from 

 geographic inquiry that the present Ozark uplift is a very recent upris- 

 ing — probably post-Tertiary. 



In the face of these facts, the hiatus in central Missouri has an unusual 

 significance. It points at once to the suggestion that the area in which 

 there was no Devonian deposition was not a subcircular belt coincident 

 with the present flanks of the Ozark dome, but a more or less linear 

 district — a narrow ridge, so to speak. The structure of the Ordovician 

 beds of the region also indicates that this is the true explanation. As 

 long ago as 1892 I called attention to the location of this old ridge. 

 Today its importance appears much greater than had previously been 

 supposed. 



Comparative Values of correlative Methods 



Not nearly enough detailed work has yet been done on the fossils to 

 enable exact correlations to be made through the faunas alone. Were 

 it not for the adoption of other and independent methods of correlation, 

 the strata of the region, so far as the paralleling of the different vertical 

 sections is concerned, might for a long time yet remain in a very unsat- 

 isfactory condition. 



In the correlation of the strata five distinct methods have been made 

 use of. In consequence the results obtained by one method have been 

 checked by those arrived at through other independent data. In this 

 way marked discrepancies in the readings of one set of records have been 

 detected and corrected. The values of the several methods have been 

 quite different in different localities, but when all could be applied in a 

 single district the comparative results have been full of interest. This 



XLIIT— Bum,. Grot,. Soc. Am., Vol. 13, 1901 



