J. Barrell — Upper Devonian Delta. 87 



Art. YII. — The Upper Devonian Delta of the Appala- 

 chian Geosyncline j * by Joseph Barrell. 



Part II. Factors Controlling the Present Limits of 

 the Strata. 



Contents. 



Introduction and. summary 87 



Present extent and thickness of the Upper Devonian 88 



Comparison of this restoration with other maps 91 



Influence of erosion cycles 93 



Present limits dependent upon intervening baselevels _ . 93 

 Relation of post-Ordovician erosion to original limits of 



Devonian .. 95 



Influence of late Paleozoic cycles of erosion 96 



Degree of destruction by the pre-Newark cycle 97 



The Newark erosion cycle and the qnartzite conglom- 

 erates 99 



The Jurassic erosion cycle . 102 



The post-Jurassic erosion cycles 103 



Absence of structural relations between present and orig- 

 inal limits r 105 



Introduction and Summary. 



Toward the west and southwest the Upper Devonian delta 

 faced the sea. The study of its strata, guided by criteria for 

 distinguishing the modes of origin of the several formations, 

 has led in the first part to certain conclusions. But facing the 

 east the marginal outcrop is near the region of maximum thick- 

 ness. The beveled edges of the strata, measured in thousands 

 of feet, bear witness to a former extension toward the ancient 

 land of Appalachia. The nature of the surface and the relations 

 of the delta to the sea are but. one half of the problem. The 

 former extent and nature of the strata now destroyed and the 

 relations of the delta to the old-land constitute the other half. 



Although Appalachia and its marginal waste plains no longer 

 exist, we may attack these questions by inferences from the 

 nature of the remaining strata, from the structure of the rocks 

 beyond, and by the evidences as to the amounts of erosion 

 which they have suffered. Merely because the study is infer- 

 ential, the results are not necessarily less secure. The com- 

 monest conclusions in geology are in fact inferential. Therefore 

 it is not the absence of inference, rather is it the definiteness 

 and convergence of independent lines of evidence and the 

 soundness of the principles according to which the conclusions 

 are inferred which determine the security of the answers. It 

 is seen then that the principles of interpretation are as impor- 



* Continued from this Journal, (4), xxxvi, pp. 429-472, 1913. 



