STRATIGRAPHIC CLASSIFICATION DIASTROPHIC CRITERIA 425 



as well as in the Ordovician period seems demonstrable, but the dis- 

 cussion of the evidence would require more space than is now available. 

 Some of the faunal data bearing on this problem are discussed in the 

 chapter on Paleontological criteria (page 485). The sequence and dis- 

 tribution of the Niagaran formations also are briefly commented on in a 

 later chapter of this part (pages 558 to 561). 



Devonian, Waverlyan and Tennesean tilting — General discussion. — In- 

 stances of local tilting in Devonian and Mississippian ages bave not been so 

 fully worked out as in the Silurian and Ordovician rocks. A glance at the 

 correlation tables in Part III, however, will show that southeastern Nortli 

 America was subjected to very considerable oscillation during at least 

 the Devonian and Waverlyan. Comparison of Helderbergian sections, in 

 the Allegheny Front and Appalachian Valle}'-, from Virginia to New 

 York, indicates north-south and east-west tilting, much the same in 

 character as occurred during the Blount stage of the Ordovician in Ten- 

 nessee (see page 545). Something of the kind is also suggested during 

 the Oriskany, and more certainly in the Mississippi Valley, during the 

 Kinderhookian and Osagian. During the middle and late Devonian and 

 most of the Tennessean the movements seem to have been broader in scope. 

 Possibly the mass of the continent had become more rigid, the vertical 

 movements more distributed, and the local elevations or islands in the 

 regions of frequent submergence less subject to warping. Still, in view 

 of the fact that the average inland extent of Neopaleozoic seas was less 

 than that of the Ordovician, indicating a greater average elevation of the 

 interior areas of the continent and consequently less complete submergence 

 of the islands and of the peninsular projections from the larger land areas, 

 it appears likely that the failure to note the differential features of the 

 vertical movements is occasioned by lack of sufficient subsidence to effect 

 submergence and deposition. Applying the idea to the Nashville dome 

 it is at least possible that the east side, on which neither Silurian or 

 Devonian sediments are found, was relatively depressed at times when the 

 western flank was distinctly emerged. In other words, that the movement 

 of the east side was relatively less effective, with respect to submergence, 

 than of the west, and that the stages of subsidence on the former were 

 never sufficient during these periods to permit submergence of parts now 

 accessible. 



Devonian and Waverlyan tilting of the Ozark uplift. — Differential 



vertical movements seem to have affected the area of the Mississippi 



Valley in a north and south direction during the middle and later 



stages of the Devonian and in the early Mississippian or Waverlyan. 



XXIX— Bull. Geol. Soc. Am., Vol. 22, 1910 



