PRINCIPLES OF STRATIGRAPHIC CORRELATIONS 565 



on the dividing axis the Fort Payne rests on the Knox dolomite, but a 

 considerable part of this hiatus is probably due to pre-Waverlyan erosion. 



The transverse, northwest-southeast axes are believed to have extended 

 across Appalachia to the sea. However, as time went on and the belt of 

 folding migrated inland their natural tendency to form peninsular pro- 

 jections between broad embayments of the shore must have grown less. 

 It is scarcely indicated by the present shoreline, but I venture to predict 

 that the close study of the Mesozoic and Cenozoic deposits under the 

 coastal plain now being carried on by Mr. T. Wayland Vaughn and asso- 

 ciates will show that during these ages the tendency of the axes to form 

 peninsulas was still well expressed. 



Bowing of the Appalachian tract. — Finally, it is a significant fact that 

 the two northwestwardly bowed parts of the Appalachian Valley tract — 

 these parts falling in the Maryland basin on the north and the Tennessee 

 basin on the south — contain by far the greatest thicknesses of Eopaleo- 

 zoic rocks. They are of great thickness also in and northeastward from 

 the Champlain Valley, where the Appalachian strike is similarly bowed. 

 And in central Alabama, close to the Cretaceous border, where the strike 

 also turns somewhat westward, the aggregate thickness of Eopaleozoic 

 deposits is much superior to their development in the three southeast- 

 wardly bowed parts of the Appalachian Valley. On the other hand, the 

 latter parts are backed immediately to the west of the valley tract by 

 enormous thicknesses of Devonian and Pennsylvanian deposits. 



This distinct distribution of the early and late Paleozoic deposits is 

 believed to be the originating cause of the sinuous trend of the Appala- 

 chian Valley. The cause is really twofold. In the first place, the 

 Eopaleozoic deposits consist largely of heavy bedded limestones ; hence of 

 material that is of more than average competence in the transmission of 

 pressure. In the second place, these competent beds attain their greatest 

 development in areas lying between other areas in which the Eopaleozoic 

 deposits are not only thinner, but in which the Devonian and Pennsyl- 

 vanian deposits in the Allegheny basins behind them are thickest. The 

 latter, therefore, are not only capable of offering extraordinary resistance, 

 but, because of the weak development of the older formations in the 

 valley tract immediately in front of them, the westward transmission of 

 pressure was here also less. In these eastwardly bowed portions by far 

 the most of the contraction of the Appalachian tract was by overth rusting. 

 The valley proper in such places is narrower than usual, being in fact en- 

 tirely concealed in the Hudson Valley. In the westwardly bowed por- 

 tions, on the contrary, the contraction was more distributed and a con- 



